const.md

const

const is a very welcomed addition offered by ES6 / TypeScript. It allows you to be immutable with variables. This is good from a documentation as well as a runtime perspective. To use const just replace var with const:

const foo = 123;

The syntax is much better (IMHO) than other languages that force the user to type something like let constant foo i.e. a variable + behavior specifier.

const is a good practice for both readability and maintainability and avoids using magic literals e.g.

// Low readability
if (x > 10) {
}

// Better!
const maxRows = 10;
if (x > maxRows) {
}

const declarations must be initialized

The following is a compiler error:

const foo; // ERROR: const declarations must be initialized

Left hand side of assignment cannot be a constant

Constants are immutable after creation, so if you try to assign them to a new value it is a compiler error:

const foo = 123;
foo = 456; // ERROR: Left-hand side of an assignment expression cannot be a constant

Block Scoped

A const is block scoped like we saw with let:

const foo = 123;
if (true) {
    const foo = 456; // Allowed as its a new variable limited to this `if` block
}

Deep immutability

A const works with object literals as well, as far as protecting the variable reference is concerned:

const foo = { bar: 123 };
foo = { bar: 456 }; // ERROR : Left hand side of an assignment expression cannot be a constant

However, it still allows sub properties of objects to be mutated, as shown below:

const foo = { bar: 123 };
foo.bar = 456; // Allowed!
console.log(foo); // { bar: 456 }

Prefer const

Always use const, unless you plan to either lazily initialization of a variable, or do a reassignment (use let for those cases).