JavaScript Type Conversion

In JavaScript, values may be of different types. Changing the type of a variable can be done by explicit type conversion. Besides that, JavaScript also performs type coercion (implicit type conversion) when the context requires it.

Type Conversion

JavaScript does not have a construct to cast into a (different) type like many other languages but some built-in helpers can be used instead. Most notably, the global objects Boolean, Number and String can be used as functions to convert a value to the respective type.

Converting to a Boolean (Truthy/Falsy Values)

With Boolean(value) you can convert any value to a boolean. How does that work?

There is a fixed set of values, so called falsy values, that convert to false. Most importantly, false, 0, empty string, null, undefined and NaN are falsy. The MDN article on "Falsy" shows the complete list.

For all other values, Boolean returns true. These values are called truthy.

Boolean(-1);
// => true

Boolean(0);
// => false

Boolean(' ');
// => true

Boolean('');
// => false
        

Note that because of the described rules, '0', 'false', [] and {} are truthy in JavaScript.

Converting to a Number

Number(value) can be used to convert a value to a number. Whitespaces at the beginning and the end of a string are ignored and an empty string is converted to 0. If you try to convert a non-primitive value or a string that does not represent a number, no error will be thrown. Instead, the result is NaN (Not-A-Number).

Number('  -12.34  ');
// => -12.34

Number('1,2');
// => NaN

Number('1 2');
// => NaN

Number('');
// => 0

Number('10e3');
// => 10000

Number({ key: '123' });
// => NaN
        

Below you can see what Number returns for other primitive values.

Number(true);
// => 1

Number(false);
// => 0

Number(null);
// => 0

Number(undefined);
// => NaN
        

Note that in contrast to the last example, Number() called without any argument is defined to return 0.

JavaScript also provides the functions parseInt and parseFloat. They apply laxer rules as to which strings can be converted to a number. Because of that, Number should be preferred as the conversion function to avoid unexpected outcomes.

parseInt('123a45');
// => 123

Number('123a45');
// => NaN
        

Converting to a String

With String(value) you can convert a value to a string. The result is what you would expect it to be for primitive values.

String(12.34);
// => '12.34'

String(false);
// => 'false'

String(null);
// => 'null'

String(undefined);
// => 'undefined'
        

For arrays, the String function will apply the string conversion for each element and join the results with a comma. You can also apply the join method yourself, e.g. to customize the separator. However, in these cases null and undefined will be converted to an empty string.

String([42, null, true, 'abc']);
// => '42,,true,abc'
        

For objects, by default String returns an unhelpful text.

String({ key: 'value' });
// => '[object Object]'
        

You can customize the conversion behavior, e.g. by providing a toString method. The section "Object to primitive conversion" on javascript.info explains the details.

Another common way to achieve a better string representation for objects and arrays is to use JSON encoding. You can convert into a string with JSON.stringify and back into an object or array with JSON.parse.

const obj = {
  name: 'Gilda Guerra',
  address: {
    city: 'Goiania',
  },
};

JSON.stringify(obj);
// => '{"name":"Gilda Guerra","address":{"city":"Goiania"}}'
        

Type Coercion

In certain contexts, JavaScript will automatically convert a value to another data type before it evaluates some statement. This implicit conversion is called type coercion.

Boolean Context

When a value is used in a boolean context, JavaScript will apply the same rules as the Boolean function to implicitly convert the value.

• When the condition of an if statement is not a boolean, coercion is applied to determine whether the condition is fulfilled or not. The same applies for the first operand of the ternary operator ?.
• The operand of the logical NOT operator ! is also coerced into boolean before the NOT operation is applied.
• JavaScript also applies coercion for the operands of the logical AND (&&) and OR (||) operators. But keep in mind that the result of the expression is not necessarily a boolean. It returns one of the original operands (see MDN on Logical Operators).

String Context

If the addition operator + is used for primitive values and one operand is a string, the other one will be coerced into a string as well. The conversion logic is the same as when using the String function. Afterwards, the two strings are concatenated.

let name;
'hello ' + name;
// => 'hello undefined'
        

The same implicit conversion happens for non-string values that are embedded in template strings.

const degrees = 23;
`It is ${degrees} °C`;
// => 'Is is 23 °C.';
        

Numeric Context

Many operators coerce the operands into numbers (if necessary) according to the logic of the Number function explained above.

• Arithmetic operators: + (if no string is involved), -, *, /, %, **
• Unary plus and unary negation operators: +, -
• Relational operators (for non-string operands): >, >=, <, <=
• Bitwise operators: |, &, ^, ~

Refer to the MDN list of operators for more details about any of those operators.

When an operand could potentially be a string, it is best to always explicitly convert with the Number function to avoid mistakes.

'1' + '2';
// => '12'
// addition operator in string context as explained above

Number('1') + Number('2');
// => 3
        

Sometimes you will see the unary plus operator being used to coerce a string into a number. This is not recommended because it is much harder to read than the explicit Number call.

const value = '42';
+value;
// => 42

Number(value);
// => 42
        

Using the loose equality and inequality operators ==/!= also often involves an implicit conversion to a number. However, the exact logic of these operators is rather complicated (see MDN on loose equality). The results are hard to predict and sometimes not what you would expect. Use the strict equality/inequality operators ===/!== instead to avoid these implicit conversions.

Note: copied from Exercism