Convert any value to a boolean in JavaScript
When you’re working with JavaScript, you often stumble upon a situation where you need to check certain conditions. And often, you would need a boolean value (true or false) or boolean expression to check for the condition.
Many times, you would deduce the boolean value from, a condition, let’s say. But what if you have a value that is not a boolean value and you need to convert it to a boolean value? For instance, you may want to safely convert a string or a number to a boolean value.
In JavaScript, there are a couple of ways to convert any value to a boolean value. Let’s see them one by one.
The Boolean() object
The first way to convert any value to a boolean value is by using the built-in Boolean() object. This function takes a value as an argument and returns a boolean value.
For instance, if you pass a string to the Boolean() object, it will return true if the string is not empty and false if the string is empty.
let strBoolValue = Boolean('foo'); // true
let strBoolValue = Boolean(''); // false
Similarly, if you pass a number to the Boolean() object, it will return true if the number is not zero and false if the number is zero.
let numberBoolValue = Boolean(1); // true
let numberBoolValue = Boolean(0); // false
If you pass an object/array to the Boolean() object, it will return true irrespective of whether the object/array is empty or not.
let objBoolValue = Boolean({}); // true
let objBoolValue = Boolean({ foo: 'bar' }); // true
let arrBoolValue = Boolean([]); // true
let arrBoolValue = Boolean([1, 2, 3]); // true
If you pass a boolean value to the Boolean() object, it will return the same boolean value.
let boolBoolValue = Boolean(true); // true
If you pass null or undefined to the Boolean() object, it will return false.
let nullBoolValue = Boolean(null); // false
let undefinedBoolValue = Boolean(undefined); // false
The Bang Bang (!!) operator
There’s another way to convert any value to a boolean value. And that is by using the Bang Bang (!!) operator. This operator is a shorthand and a more convenient way to infer a boolean value from a value.
It works the same way as the Boolean() object under the hood. So, all the previous examples will work the same way with the !! operator.
let strBoolValue = !!'foo'; // true
let strBoolValue = !!''; // false
let numberBoolValue = !!1; // true
let numberBoolValue = !!0; // false
let objBoolValue = !!{}; // true
let objBoolValue = !!{ foo: 'bar' }; // true
let arrBoolValue = !![]; // true
let arrBoolValue = !![1, 2, 3]; // true
let boolBoolValue = !!true; // true
let nullBoolValue = !!null; // false
let undefinedBoolValue = !!undefined; // false
Converting “true”/“false” to boolean
This is a bonus use case and may not be in the scope of this article. But, I thought it would be a good idea to mention it here.
So, one problem with the above methods is that they don’t convert the string "true" or "false" to their respective boolean values.
For instance, if you pass the string "false" to the Boolean() function, it will still return true instead of false.
To get around this, you can use the JSON.parse() function to convert the string to a boolean value like so.
let yesBoolValue = JSON.parse('true'); // true
let noBoolValue = JSON.parse('false'); // false
Just make sure you don’t pass any other values or empty strings to the JSON.parse() function as it will throw an error.
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