Explicit Coercion

If we want to change a data type, such as a string of numbers to a Number type, we perform an explicit type coercion.

  • String to Numbers

Use the Number function to coerce a string to a number:

> Number('1')
= 1

The Number function accepts a string value argument, returning a number if the string is comprised of valid numeric data.

If the string doesn't have valid numeric data, Number() will return NaN

Coercing an empty string to a number returns a 0

Coercing a boolean value from true returns a 1 while coercing false returns a 0

  • parseInt() function alternative

Similar to the Number() function for coercion but works with a mix of string data comprised of numbers and non-numeric characters.

parseInt() will convert string numbers until it reaches an invalid character.

> parseInt('12xyz')
= 12

Note that if you pass a fractional character to parseInt() it will only return the integer value, not any decimal values less than 1.

> parseInt('3.1415')
= 3

parseInt() can accept a second argument called the radix; this indicates the base of the number argument, i.e.: radix of 2 = base 2

  • parseFloat() alternative

However, the parseFloat() function will handle fractional numbers for explicit coercion.

> parseFloat('12.5foo')
= 12.5
  • Numbers to strings

To explicitly coerce numbers into a string data type, use the String() function.

This function can be used on undefined and null while the toString() can not.

> String(20)
= '20'
  • Using the unary + operator to coerce strings to numbers
+'1' // returns the number value 1
  • Coercing values to strings with the toString() method
let number = 123;
console.log(number.toString()); // returns the string "123"

This can also be used on arrays:

[1, 2, 3].toString(); // returns the concatenated string "1, 2, 3";

Implicit Coercion

  • Using the == operator

The non-strict equality operator will coerce values that are not of the same type:

  1. When a number is compared with a string using ==, the string is coerced into a number.
  2. When a boolean is compared with any other value, it is coerced into a number and compared again using the == operator.
  3. When an object is compared with a primitive value, the object is coerced into a primitive value and compared again using the == operator.
  4. A == comparison of undefined with null evaluates as true.

When using the non-strict equality operator on two objects, it only returns true if they are the same object.

  • Using the binary + operator

When one of the operands is a string, the + operator will coerce the other operand to a string and concatenate them.

console.log('number ' + 1); // returns the string 'number 1'

When one of the operands is an object, both are converted to strings and concatenated:

[1] + 2;        // "12"
[1] + '2';      // "12"
[1, 2] + 3;     // "1,23"
[] + 5;         // "5"
[] + true;      // "true"
42 + {};        // "42[object Object]"