How can I convert a string to boolean in JavaScript?

Do:

var isTrueSet = (myValue === 'true');

using the identity operator (===), which doesn't make any implicit type conversions when the compared variables have different types.

Don't:

You should probably be cautious about using these two methods for your specific needs:

var myBool = Boolean("false");  // == true

var myBool = !!"false";  // == true

Any string which isn't an empty string will evaluate to true by using them. Although they're the cleanest methods I can think of concerning javascript boolean conversion, I think they're not what you're looking for.

Remember to match case:

var isTrueSet = (myValue.toLowerCase() === 'true');

Also, if it's a form element checkbox, you can also detect if the checkbox is checked:

var isTrueSet = document.myForm.IS_TRUE.checked;

Assuming that if it is checked, it is "set" equal to true. This evaluates as true/false.

Using test()

let stringValue = "true";

let boolValue = (/true/i).test(stringValue) //returns true

test() method of javascript matches a regular expression against a string.
In this case we check whether the string contains true or not. Now, if the string value is ‘true’, then boolValue will be true, else a false./i at the end of regular expression is for case insensitive match.

Using switch-case
A switch-case construct is used to cover all possible string value combinations which should lead to a true boolean value.
These string values may be ‘true’, ‘1’, ‘on’, ‘yes’. It also covers boolean true or numeric 1.

If the given string (‘true’) or any other values match, then the boolean true is returned while boolean false is returned in every other case.

let stringValue = "true";
let boolValue = getBoolean(stringValue); //returns true

function getBoolean(value){
	switch(value) {
		case true:
		case "true":
		case 1:
		case "1":
		case "on":
		case "yes":
		return true;
		default:
		return false;
	}
}