diff --git a/bin/heta-build.js b/bin/heta-build.js
index dbf44554..f5fdc9ea 100644
--- a/bin/heta-build.js
+++ b/bin/heta-build.js
@@ -1,7 +1,7 @@
 #!/usr/bin/env node
 const { Command } = require('commander');
 const program = new Command();
-const fs = require('fs');
+const fs = require('fs-extra');
 const path = require('path');
 const { Builder } = require('../src');
 const YAML = require('js-yaml'); // https://www.npmjs.com/package/js-yaml
@@ -115,8 +115,12 @@ async function main() {
     process.exit(2); // BRAKE
   }
 
-  let builder = new Builder(declaration, targetDir);
-  builder.run();
+  let builder = new Builder(
+    declaration,
+    targetDir,
+    fs.readFileSync,
+    fs.outputFileSync
+  ).run();
 
   return builder;
 }
diff --git a/src/builder/index-online.js b/src/builder/index-online.js
new file mode 100644
index 00000000..73a5487e
--- /dev/null
+++ b/src/builder/index-online.js
@@ -0,0 +1,149 @@
+function build(inputDict, settings) { // modules, exports
+    let coreDirname = '/';
+    /*
+        constructor()
+    */
+
+    // create container and logger
+    let c = new Container();
+    
+    /*
+    c.logger.addTransport((level, msg, opt, levelNum) => { // temporal solution, all logs to console
+        console.log(`{heta-compiler} [${level}]\t${msg}`);
+    });
+    */
+    let minLogLevel = settings.options?.logLevel || 'info';
+    let minLevelNum = levels.indexOf(minLogLevel);
+    c.logger.addTransport((level, msg, opt, levelNum) => {
+        let value = `\n[${level}]\t${msg}`;
+        if (levelNum >= minLevelNum) {
+            postMessage({action: 'console', value: value});
+        }
+    });
+
+    // file paths
+    let _coreDirname = path.resolve(coreDirname);
+    let _distDirname = path.resolve(coreDirname, settings.options.distDir);
+    let _metaDirname = path.resolve(coreDirname, settings.options.metaDir);
+    let _logPath = path.resolve(coreDirname, settings.options.logPath);
+
+    c.logger.info(`Heta builder of version ${hetaCompilerPackage.version} initialized in directory "${_coreDirname}"`);
+    if (settings.id) c.logger.info(`Platform id: "${settings.id}"`);
+
+    /*
+        run()
+    */
+    let outputDict = {}; // {<filepath>: <Buffer>}
+    c.logger.info(`Compilation of module "${settings.importModule.source}" of type "${settings.importModule.type}"...`);
+
+    // 1. Parsing
+    let ms = new ModuleSystem(c.logger, (filename) => {
+        let arrayBuffer = inputDict[filename]; // Uint8Array
+        if (!arrayBuffer) {
+            throw new HetaLevelError(`Module ${filename} is not found.`);
+        }
+        let buffer = Buffer.from(arrayBuffer); // Buffer
+        
+        return buffer;
+    });
+    let sourceFilepath = path.resolve(_coreDirname, settings.importModule.source);
+    let sourceType = settings.importModule.type;
+    ms.addModuleDeep(sourceFilepath, sourceType, settings.importModule);
+
+    // 2. Modules integration
+    if (settings.options.debug) {
+        Object.getOwnPropertyNames(ms.moduleCollection).forEach((name) => {
+            let relPath = path.relative(_coreDirname, name + '.json');
+            let absPath = path.join(_metaDirname, relPath);
+            let str = JSON.stringify(ms.moduleCollection[name], null, 2);
+            outputDict[absPath] = Buffer.from(str, 'utf-8');
+            c.logger.info(`Meta file was saved to ${absPath}`);
+        });
+    }
+    let qArr = ms.integrate();
+    
+    // 3. Translation
+    c.loadMany(qArr, false);
+    
+    // 4. Binding
+    c.logger.info('Setting references in elements, total length ' + c.length);
+    c.knitMany();
+    
+    // 5. Circular start_ and ode_
+    c.logger.info('Checking for circular references in Records.');
+    c.checkCircRecord();
+    
+    // 6. check circ UnitDef
+    c.checkCircUnitDef();
+
+    
+    // === STOP if errors ===
+    if (!c.logger.hasErrors) {
+
+        // 7. Units checking
+        if (settings.options.unitsCheck) {
+          c.logger.info('Checking unit\'s consistency.');
+          c.checkUnits();
+        } else {
+          c.logger.warn('Units checking skipped. To turn it on set "unitsCheck: true" in declaration.');
+        }
+
+        // 8. Terms checking
+        c.logger.info('Checking unit\'s terms.');
+        c.checkTerms();
+
+        // 9. Exports
+        // save
+        if (settings.options.skipExport) {
+            c.logger.warn('Exporting skipped as stated in declaration.');
+        } else if (settings.options.juliaOnly) {
+            c.logger.warn('"Julia only" mode');
+            //this.exportJuliaOnly(); 
+            // create export without putting it to exportStorage
+            let Julia = this.container.classes['Julia'];
+            let exportItem = new Julia({
+                format: 'Julia',
+                filepath: '_julia'
+            });
+
+            _makeAndSave(exportItem, _distDirname, outputDict);
+        } else {
+            //this.exportMany();
+            let exportElements = [...c.exportStorage].map((x) => x[1]);
+            c.logger.info(`Start exporting to files, total: ${exportElements.length}.`);
+
+            exportElements.forEach((exportItem) => _makeAndSave(exportItem, _distDirname, outputDict));
+        }
+      } else {
+        c.logger.warn('Units checking and export were skipped because of errors in compilation.');
+      }
+
+    // 10. save logs if required
+    let hetaErrors = c.hetaErrors();
+    let createLog = settings.options.logMode === 'always' 
+      || (settings.options.logMode === 'error' && hetaErrors.length > 0);
+    if (createLog) {
+      switch (settings.options.logFormat) {
+      case 'json':
+        var logs = JSON.stringify(c.defaultLogs, null, 2);
+        break;
+      default: 
+        logs = c.defaultLogs
+          .filter(x => x.levelNum >= minLevelNum)
+          .map(x => `[${x.level}]\t${x.msg}`)
+          .join('\n');  
+      }
+
+      //fs.outputFileSync(_logPath, logs);
+      outputDict[_logPath] = Buffer.from(logs, 'utf-8'); // Buffer
+      c.logger.info(`All logs was saved to file: "${_logPath}"`);
+    }
+
+    if (!c.logger.hasErrors) {
+        postMessage({action: 'finished', dict: outputDict});
+    } else {
+        postMessage({action: 'stop', dict: outputDict});
+    }
+    
+    return c;
+}
\ No newline at end of file
diff --git a/src/builder/index.js b/src/builder/index.js
index fc954cb9..a50ef403 100644
--- a/src/builder/index.js
+++ b/src/builder/index.js
@@ -41,8 +41,8 @@ class Builder {
   constructor(
     declaration = {},
     coreDirname = '.',
-    fileReadHandler = (fn) => fs.readFileSync(fn), // return text
-    fileWriteHandler = (fn, text) => fs.outputFileSync(fn, text) // return undefined
+    fileReadHandler = (fn) => { throw new Error('File read is not set for Builder'); }, // return text
+    fileWriteHandler = (fn, text) => { throw new Error('File write is not set for Builder'); } // return undefined
   ) {
     // create container
     this.container = new Container();
@@ -197,7 +197,8 @@ class Builder {
       this.fileWriteHandler(this._logPath, logs);
       this.logger.info(`All logs was saved to file: "${this._logPath}"`);
     }
-    return;
+
+    return this;
   }
 
   /**
diff --git a/test/builder/index.js b/test/builder/index.js
index b2e8a674..6539b247 100644
--- a/test/builder/index.js
+++ b/test/builder/index.js
@@ -2,11 +2,12 @@
 const { Builder } = require('../../src/builder');
 const declaration = require('./test-platform');
 const { expect } = require('chai');
+const fs = require('fs-extra');
 
 describe('Test Builder.', () => {
   let b;
   it('Create Builder object and run.', () => {
-    b = new Builder(declaration, __dirname);
+    b = new Builder(declaration, __dirname, fs.readFileSync, () => {});
     b.run();
   });
 });
diff --git a/test/cases/0.js b/test/cases/0.js
index 30c9302a..0ea02506 100644
--- a/test/cases/0.js
+++ b/test/cases/0.js
@@ -15,7 +15,7 @@ use(chaiXml);
 const { load } = require('js-yaml');
 const fs = require('fs-extra');
 const { slvParse } = require('slv-utils');
-const XLSX = require('xlsx'); 
+const XLSX = require('xlsx');
 
 const sbml_l2v4_correct = fs.readFileSync('cases/0-hello-world/master/mm_sbml_l2v4/mm.xml','utf8');
 const sbml_l3v1_correct = fs.readFileSync('cases/0-hello-world/master/mm_sbml_l3v1/mm.xml','utf8');
@@ -52,7 +52,7 @@ describe('Testing "cases/0-hello-world"', () => {
         {format: 'Mrgsolve', spaceFilter: '^mm$'},
         {format: 'Julia', spaceFilter: '^mm$'}]
     };
-    b = new Builder(declaration, 'cases/0-hello-world');
+    b = new Builder(declaration, 'cases/0-hello-world', fs.readFileSync, () => {});
   });
 
   it('Run include', () => {
diff --git a/test/cases/12.js b/test/cases/12.js
index b1eb6042..f844999e 100644
--- a/test/cases/12.js
+++ b/test/cases/12.js
@@ -3,9 +3,7 @@ const { Builder } = require('../../src/builder');
 const { expect, use } = require('chai');
 const chaiXml = require('chai-xml');
 use(chaiXml);
-//const { load } = require('js-yaml');
 const fs = require('fs-extra');
-//const { slvParse } = require('slv-utils');
 
 const sbml_correct = fs.readFileSync('cases/12-to-sbml/master/sbml/first.xml','utf8');
 const json_correct = require('../../cases/12-to-sbml/master/output.json');
@@ -25,7 +23,7 @@ describe('Testing "cases/12-to-sbml"', () => {
         source: 'src/index.heta'
       }
     };
-    b = new Builder(declaration, 'cases/12-to-sbml');
+    b = new Builder(declaration, 'cases/12-to-sbml', fs.readFileSync, () => {});
     //console.log(b);
   });
 
diff --git a/test/cases/14.js b/test/cases/14.js
index 217049ee..2f418073 100644
--- a/test/cases/14.js
+++ b/test/cases/14.js
@@ -2,6 +2,7 @@
 
 const { Builder } = require('../../src');
 const { expect, use } = require('chai');
+const fs = require('fs-extra');
 
 const json_correct = require('../../cases/14-sbml-module/master/output.json');
 
@@ -22,7 +23,7 @@ describe('Case #14: testing SBML module with units', () => {
       export: [
         {format: 'JSON'}
       ]
-    }, 'cases/14-sbml-module');
+    }, 'cases/14-sbml-module', fs.readFileSync, () => {});
     b.run();
     expect(b.container.logger).to.have.property('hasErrors').false;
   });
diff --git a/test/cases/6.js b/test/cases/6.js
index b2b11824..91eedff2 100644
--- a/test/cases/6.js
+++ b/test/cases/6.js
@@ -28,7 +28,7 @@ describe('Testing "cases/6-import"', () => {
         {format: 'SBML', spaceFilter: 'model'},
       ]
     };
-    b = new Builder(declaration, 'cases/6-import');
+    b = new Builder(declaration, 'cases/6-import', fs.readFileSync, () => {});
     //console.log(b);
   });
 
diff --git a/test/cases/7.tmp.js b/test/cases/7.tmp.js
index 47987bb4..ffef6a8d 100644
--- a/test/cases/7.tmp.js
+++ b/test/cases/7.tmp.js
@@ -24,7 +24,7 @@ describe('Testing "cases/7-importNS"', () => {
         source: 'src/index.heta'
       }
     };
-    b = new Builder(declaration, 'cases/7-importNS');
+    b = new Builder(declaration, 'cases/7-importNS', fs.readFileSync, () => {});
     //console.log(b);
   });
 
diff --git a/test/module-system/index.js b/test/module-system/index.js
index d0b13802..f09fd860 100644
--- a/test/module-system/index.js
+++ b/test/module-system/index.js
@@ -3,7 +3,6 @@ const { expect } = require('chai');
 const ModuleSystem = require('../../src/module-system');
 const { Logger } = require('../../src/logger');
 const path = require('path');
-//const { writeFileSync } = require('fs');
 const expected = require('./expected');
 const noImportOutput = require('./no-include');
 const fs = require('fs');
@@ -26,7 +25,6 @@ describe('Run normal ModuleSystem.', () => {
     let ms = new ModuleSystem(logger, (filename) => fs.readFileSync(filename));
     let filepath = path.join(__dirname, './normal-a.heta');
     let mdl = ms.addModuleDeep(filepath, 'heta', {});
-    //writeFileSync('res0-new.json', JSON.stringify(ms, null, 2));
 
     expect(mdl).to.be.with.a('Array').lengthOf(3);
 
@@ -34,7 +32,6 @@ describe('Run normal ModuleSystem.', () => {
     expect(Object.keys(ms.graph.map)).to.have.property('length', 5);
 
     let arr = ms.integrate();
-    //writeFileSync('res-new.json', JSON.stringify(arr, null, 2));
     expect(arr).to.be.deep.equal(expected);
   });
 });
diff --git a/test/module-xlsx/index.js b/test/module-xlsx/index.js
index 06018b07..047ca644 100644
--- a/test/module-xlsx/index.js
+++ b/test/module-xlsx/index.js
@@ -3,6 +3,7 @@ const { expect } = require('chai');
 const { Builder } = require('../../src/builder');
 const outputNameless = require('./output-nameless');
 const outputOne = require('./output-one');
+const fs = require('fs-extra');
 
 describe('Integral test of correct xlsx module', () => {
   it('Create platform for single XLSX sheet.', () => {
@@ -18,7 +19,7 @@ describe('Integral test of correct xlsx module', () => {
         omitRows: 2
       }
     };
-    let b = new Builder(declaration, __dirname);
+    let b = new Builder(declaration, __dirname, fs.readFileSync, () => {});
     b.run();
     let resultNameless = b.container.namespaceStorage.get('nameless').toQArr(true);
     expect(resultNameless).to.be.deep.equal(outputNameless);
@@ -38,7 +39,7 @@ describe('Integral test of correct xlsx module', () => {
         sheet: 1
       }
     };
-    let b = new Builder(declaration, __dirname);
+    let b = new Builder(declaration, __dirname, fs.readFileSync, () => {});
     b.run();
     let resultNameless = b.container.namespaceStorage.get('nameless').toQArr(true);
     expect(resultNameless).to.be.deep.equal(outputNameless);
@@ -56,7 +57,7 @@ describe('Integral test of correct xlsx module', () => {
         sheet: 9
       }
     };
-    let b = new Builder(declaration, __dirname);
+    let b = new Builder(declaration, __dirname, fs.readFileSync, () => {});
     
     b.run();
     expect(b.container.hetaErrors()).to.be.lengthOf(1);