Check if every element of array satisfies a condition in JavaScript

In JavaScript, Array.every() function is used to check if every element of array satisfies a given criteria represented by a function.

Syntax

array.every( function, thisValue )

• array is the given array.
• every is inbuilt array method name.
• function a function to run for every element. the definition of the function is function(element, index, arr). the function must return a boolean value of true or false.
  • element is the current element in the array.
  • index of the current element [optional argument].
  • arr is the array of current element [optional argument].
• this is a value passed to the function as its this value [optional argument].

every() method returns true if all the elements of the array return true for the given function. If there is at least one element that returns false, then every() method returns false for the array.

Examples

1. Check if all the strings in the given array fruits are non-empty.

function isNonEmptyString(x) {
    if (x.length != 0) {
        return true;
    } else {
        return false;
    }
}

let fruits = ['apple', 'banana', 'cherry'];
let output = fruits.every(isNonEmptyString);
console.log(output);

2. fruits have an empty string. Check if the output of every() when the isNonEmptyString() function returns false for an empty string.

function isNonEmptyString(x) {
    if (x.length != 0) {
        return true;
    } else {
        return false;
    }
}

let fruits = ['apple', '', 'cherry'];
let output = fruits.every(isNonEmptyString);
console.log(output);

3. Check if all the numbers in array are even.

function isEven(num) {
    if (num % 2 == 0) {
        return true;
    } else {
        return false;
    }
}

let numbers = [4, 10, 8, 18];
let output = numbers.every(isEven);
console.log('Are all numbers even? : ' + output);