arrays.md

Literals

Use the literal syntax for array creation.

ESLint: no-array-constructor

Examples

⇣ Incorrect code for this rule:

const snow = new Array();

⇡ Correct code for this rule:

const snow = [];

Push

Use Array#push instead of direct assignment to add items to an array.

Examples

⇣ Incorrect code for this rule:

const winter = [];

winter[snow.length] = "snow";

⇡ Correct code for this rule:

const winter = [];

winter.push("snow");

ES6 Array Spreads

Use array spreads ... to copy arrays.

Examples

⇣ Incorrect code for this rule:

const winter = ["snow", "frost"];
const winterCopy = [];
let idx;

for (idx = 0; idx < winter.length; idx += 1) {
  winterCopy[idx] = winter[idx];
}

⇡ Correct code for this rule:

const winter = ["snow", "frost"];
const winterCopy = [...winter];

Prefer spreads over from

Use spreads ... instead of Array.from to convert an array-like object to an array.

Examples

⇜ Avoidable code for this rule:

const winter = document.querySelectorAll(".snow");

const nodes = Array.from(winter);

⇢ Recommended code for this rule:

const winter = ["snow", "frost"];
const nodes = [...winter];

Mapping Over Iterables

Use Array.from instead of spread ... for mapping over iterables. This avoids creating an intermediate array.

Examples

⇣ Incorrect code for this rule:

const winter = ["snow", "frost"];
const winterMap = [...winter].map(seasons);

⇡ Correct code for this rule:

const winter = ["snow", "frost"];
const winterMap = Array.from(winter, seasons);

Callback Return

Use return statements in array method callbacks. This excludes functions that consists of a single statement returning an expression without side effects.

ESLint: array-callback-return

Examples

⇣ Incorrect code for this rule:

// No returned value means `winter` becomes undefined after the first iteration.
[
  [0, 1],
  [2, 3],
  [4, 5],
].reduce((winter, element, index) => {
  const flatten = winter.concat(element);
  winter[index] = flatten;
});

seasons.filter((season) => {
  const { name, element } = season;
  if (name === "winter") {
    return element === "snow";
  } else {
    return false;
  }
});

⇡ Correct code for this rule:

[1, 2, 3].map((num) => {
  const snow = num + 1;
  return num * snow;
});

[1, 2, 3].map((num) => num + 1);

[
  [0, 1],
  [2, 3],
  [4, 5],
].reduce((winter, element, index) => {
  const flatten = winter.concat(element);
  return flatten;
});

seasons.filter((season) => {
  const { name, element } = season;
  if (name === "winter") {
    return element === "snow";
  }

  return false;
});

Line Breaks

Use line breaks after open and before close array brackets if an array has multiple lines.

Examples

⇣ Incorrect code for this rule:

const arr = [
  [0, 1], [2, 3], [4, 5]
];

const objectInArray = [{
  id: 1
}, {
  id: 2
}];

const numberInArray = [
  1, 2
];

⇡ Correct code for this rule:

const arr = [
  [0, 1],
  [2, 3],
  [4, 5],
];

const objectInArray = [
  {
    id: 1,
  },
  {
    id: 2,
  },
];

const numberInArray = [
  1,
  2
];