Table of Contents

• Overview
• Test if an array is empty
• Concatenate arrays
• Convert an array to an object
• Test if an array contains an element
• Array.prototype.sort() Sorting Stability in Different Browsers

Overview

• Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of.
• Use map() / every() / filter() / find() / findIndex() / reduce() / some() / …

Test if an array is empty howto

• Just use array.length;

if (typeof array !== 'undefined' && array.length > 0) {
  // the array is defined and has at least one element
}

Concatenate arrays howto

[].concat(...arrays);

• if arrays > ~150000, this can fail. In this case you should use a loop-based approach.

Convert an array to an object howto

function arrayToObject(arr, fk, fv) {
  return arr.reduce((obj, x) => {
    const obj1 = obj; // avoid modifying 'obj' parameter.
    obj1[fk(x)] = fv(x);
    return obj1;
  }, {});
}

Test if an array contains an element howto

• Use Array.prototype.includes()

let pets = ['cat', 'dog', 'bat'];
console.log(pets.includes('cat'));
console.log(pets.includes('at'));

true
false

Array.prototype.sort() Sorting Stability in Different Browsers discussion

Some cursory results:

• IE6+: stable
• Firefox < 3: unstable
• Firefox >= 3: stable
• Chrome <= 5 (i.e., all versions to date): unstable
• Opera < 10: unstable
• Opera >= 10: stable
• Safari 4: stable