The every() method in JavaScript is used to test whether all elements in an array pass the test implemented by the provided callback function. It returns a boolean value: true if all elements satisfy the condition, and false otherwise.

Syntax:

let result = array.every(callback(currentValue, index, array), thisArg)

• callback: A function that is called for each element in the array. It can accept up to three parameters:

  • currentValue: The current element being processed in the array.
  • index (optional): The index of the current element being processed.
  • array (optional): The array every() was called upon.

• thisArg (optional): A value to use as this when executing the callback function.

Return Value:

• A boolean value (true or false):
  • true: If the callback function returns a truthy value for all elements in the array.
  • false: If the callback function returns a falsy value for at least one element in the array.

Key Points:

• Stops processing on the first falsy return: Once a falsy value is found, every() stops checking the remaining elements and returns false.
• Does not modify the original array: The every() method does not alter the original array; it only checks the values based on the callback function.
• Commonly used for validation: It is often used to ensure that all elements in an array meet certain criteria.

Example 1: Basic usage (Checking for positive numbers)

let numbers = [1, 2, 3, 4, 5];
let allPositive = numbers.every(function(number) {
  return number > 0;  // Check if all numbers are greater than 0
});

console.log(allPositive);  // true

Example 2: Using an arrow function (Checking for an object property)

let users = [
  { id: 1, name: 'Alice', active: true },
  { id: 2, name: 'Bob', active: true },
  { id: 3, name: 'Charlie', active: true }
];

let allActive = users.every(user => user.active);  // Check if all users are active

console.log(allActive);  // true

Example 3: Returning false when any match fails

let numbers = [1, 2, 3, -4, 5];
let allPositive = numbers.every(function(number) {
  return number > 0;  // Check if all numbers are greater than 0
});

console.log(allPositive);  // false

Example 4: Using index and array parameters

let numbers = [2, 4, 6, 8];
let allEven = numbers.every(function(element, index) {
  return element % 2 === 0;  // Check if all elements are even
});

console.log(allEven);  // true

Example 5: Using thisArg

let threshold = {
  max: 10
};

let numbers = [3, 5, 7, 9];
let allUnderThreshold = numbers.every(function(num) {
  return num < this.max;  // Check if all numbers are less than threshold.max
}, threshold);

console.log(allUnderThreshold);  // true

Summary:

• The every() method is a useful way to validate that all elements in an array meet specific criteria defined in a callback function.
• It returns a boolean indicating whether all elements satisfy the condition, making it useful for checks and validations within collections of data.
• The method is particularly effective when working with arrays of objects, allowing for straightforward queries based on object properties.