How to check if a variable is an array in JavaScript?
In JavaScript, we can check if a variable is an array by using 3 methods, using the isArray method, using the instanceof operator and using checking the constructor type if it matches an Array object.
- Method 1: Using the isArray method
The Array.isArray() method checks whether the passed variable is an Array object.
Syntax:
Array.isArray(variableName)
It returns a true boolean value if the variable is an array and false if it is not. This is shown in the example below.
Example-1:
<!DOCTYPE html><htmllang="en"><head><title>How to check if a variableis an array in JavaScript?</title></head><body><h1style="color: green">GeeksforGeeks</h1><b>How to check if a variableis an array in JavaScript?</b><p>Click on the button to checkif the variable is an array</p><p>Output for string:<divclass="outputString"></div><p>Output for number:<divclass="outputNumber"></div><p>Output for array:<divclass="outputArray"></div><buttononclick="checkArray()">Click here</button><scripttype="text/javascript">function checkArray() {let str = 'This is a string';let num = 25;let arr = [10, 20, 30, 40];ans = Array.isArray(str);document.querySelector('.outputString').textContent = ans;ans = Array.isArray(num);document.querySelector('.outputNumber').textContent = ans;ans = Array.isArray(arr);document.querySelector('.outputArray').textContent = ans;}</script></body></html>Output:
- Method 2: Using the instanceof operator
The instanceof operator is used to test whether the prototype property of a constructor appears anywhere in the prototype chain of an object. This can be used to evaluate if an the given variable has a prototype of ‘Array’.Syntax:
variable instanceof Array
The operator returns a true boolean value if the variable is same as what is specified (here an Array) and false if it is not. This is shown in the example below.
Example-2:
<!DOCTYPE html><htmllang="en"><head><title>How to check if a variable isan array in JavaScript?</title></head><body><h1style="color: green">GeeksforGeeks</h1><b>How to check if a variable isan array in JavaScript?</b><p>Click on the button to checkif the variable is an array</p><p>Output for string:<divclass="outputString"></div><p>Output for number:<divclass="outputNumber"></div><p>Output for array:<divclass="outputArray"></div><buttononclick="checkArray()">Click here</button><scripttype="text/javascript">function checkArray() {let str = 'This is a string';let num = 25;let arr = [10, 20, 30, 40];ans = str instanceof Array;document.querySelector('.outputString').textContent =ans;ans = num instanceof Array;document.querySelector('.outputNumber').textContent =ans;ans = arr instanceof Array;document.querySelector('.outputArray').textContent =ans;}</script></body></html>Output:
- Method 3: Checking the constructor property of the variable
Another method to check a variable is an array is by checking it’s constructor with Array.
Syntax:
variable.constructor === Array
This becomes true if the variable is same as what is specified (here an Array) and false if it is not. This is shown in the example below.
Example-3:
<!DOCTYPE html><htmllang="en"><head><title>How to check if a variable isan array in JavaScript?</title></head><body><h1style="color: green">GeeksforGeeks</h1><b>How to check if a variable isan array in JavaScript?</b><p>Click on the button to checkif the variable is an array</p><p>Output for string:<divclass="outputString"></div><p>Output for number:<divclass="outputNumber"></div><p>Output for array:<divclass="outputArray"></div><buttononclick="checkArray()">Click here</button><scripttype="text/javascript">function checkArray() {let str = 'This is a string';let num = 25;let arr = [10, 20, 30, 40];ans = str.constructor === Array;document.querySelector('.outputString').textContent = ans;ans = num.constructor === Array;document.querySelector('.outputNumber').textContent = ans;ans = arr.constructor === Array;document.querySelector('.outputArray').textContent = ans;}</script></body></html>Output:
JavaScript is best known for web page development but it is also used in a variety of non-browser environments. You can learn JavaScript from the ground up by following this JavaScript Tutorial and JavaScript Examples.
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