How to Set an Object Key by Variable in JavaScript

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To set an object key by variable in JavaScript, you can use either “dot notation” or “square bracket” notation.

Here’s an example using both notations:

const mainObj = {};
const key = 'mainKey';
const value = 'mainValue';

// Using dot notation
mainObj[key] = value;

// Using square bracket notation
mainObj.key2 = 'mainValue2';

console.log(mainObj);

Output

{ mainKey: 'mainValue', key2: 'mainValue2' }

In this code example, we created a new empty object mainObj.

A variable key is assigned the string value ‘mainKey‘, and a variable value is assigned the string value ‘mainValue’.

The key variable is used to set the value of a property on ‘mainObj’ using square bracket notation (mainObj[key] = value), which dynamically sets the object key based on the value of the key.

Another key2 is set using dot notation (mainObj.key2 = ‘mainValue2’), which sets the object key to the literal string ‘key2’.

Finally, the mainObj object is logged into the console.

The dot notation only works if the key name is a valid identifier, which means it must start with a letter, underscore, or dollar sign and may contain letters, digits, underscores, or dollar signs.

You must use square bracket notation if the key name is not a valid identifier. Square bracket notation is also required to use a variable to set the key name dynamically.

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