prefer-const

July 10, 2018 ยท View on GitHub

:wrench: fixable :nut_and_bolt: configurable

Enforces the use of const for variables that are never reassigned.

Rationale

Using const can significantly improve type inference and type checking by TypeScript. JavaScript runtimes are able to optimize and potentially inline constants.

Examples

:thumbsdown: Examples of incorrect code

function fn() {
  let foo = 1, bar = 2; // error on 'foo' even though 'bar' is reassigned
  var baz = 3; // error on 'baz' as it's never reassigned

  bar = 4;
}

for (let element of []) { // 'element' is never reassigned
}

:thumbsup: Examples of correct code

// global script, variables could be changed from everywhere
let foo = 1;
var bar = 2;

export {}; // make this as ES2015 module
const foo = 1;
const bar = 2;
let baz = 3; // reassigned later
baz = 4;
let bas: number; // no initializer

for (let i = 0, max = 10; i < max; ++i) { // special handling of for-loops: because 'i' is reassigned 'max' has no error
  console.log(i);
}

Configuration

This rule allows configuration on how it treats destructuring where one variable can be const while at least one other variable of the same destructuring is not a constant.

let {a, b} = {a: 1, b: 2}; // 'a' is never reassigned, 'b' is reassigned
b = 3;

Because splitting destructuring into multiple declarations is not always possible the default configuration is the more permissive all option.

destructuring: all (default)

Ignore destructuring where at least one variable is not a constant.

rules:
  prefer-const: error

Or

rules:
  prefer-const:
    options:
      destructuring: all

:thumbsdown: Examples of incorrect code with this option enabled

let {a, b} = {a: 1, b: 2}; // 'a' and 'b' are never reassigned

:thumbsup: Examples of correct code with this option enabled

let {a, b} = {a: 1, b: 2}; // 'b' is reassigned, therefore 'a' is ignored
b = 3;

destructuring: any

Add failures for any destructuring variable that can be const even if other variables are reassigned.

rules:
  prefer-const:
    options:
      destructuring: any

:thumbsdown: Examples of incorrect code with this option enabled

let {a, b} = {a: 1, b: 2}; // 'a' and 'b' are never reassigned
var {c, d} = {c: 3, d: 4}; // 'c' is never reassigned
d = 5;

:thumbsup: Examples of correct code with this option enabled

const {a, b} = {a: 1, b: 2}; // 'a' and 'b' are never reassigned

Further Reading