JavaScript Coercion

A type can be converted in 2 different ways:

• ▪ Type casting: explicitly converting a value from 1 type to another. Occurs in statically typed languages at compile time. TypeScript example: const myVar = otherVar as string.
• ▪ Coercion: occurs in dynamically typed languages, like JavaScript, at runtime. It can happen in 2 different ways:
  • ▫ explicit: it's obvious the intent is to convert a value from one type to another.
  • ▫ implicit: type conversion occurs as a side-effect of some operation.

Example, coercing a number into a string:

const a = 42;

// explicit coercion
const c = String(a);

// implicit coercion
const b = a + "";

Whenever coercion occurs, 1 or more internal operations, known as abstract operations, are performed. It always results in a primitive.

Explicit Coercion

To explicitly coerce a value to a string, number or boolean, use the built-in native contructors. The new keyword isn't used (so an object wrapper isn't created).

const a = 1;
const b = String(a);

const c = "1";
const d = Number(c);

const e = "1";
const f = Boolean(c);

These constructors can coerce any value to a primitive based on the rules of the abstract operations:

• ▪ ToString
• ▪ ToNumber
• ▪ ToBoolean

ToString

The ToString abstract operation rules are:

• ▪ null becomes "null"
• ▪ undefined becomes "undefined"
• ▪ true becomes "true"
• ▪ number: 1 becomes "1". Very small or large numbers become their exponent form:

const a = 1.07 * 1000 * 1000 * 1000 * 1000 * 1000 * 1000 * 1000;
const aStr = a.toString();

// seven times three digits => 21 digits
console.log(aStr)

Object values have their own toString() method. If you implicitly coerce an object, its toString() will automatically be called.

const obj = [1,2];

// explicit
console.log(obj.toString())

// implicit 
// if either operand to '+' is a string, the operation will be string concatenation
console.log(obj + "")

ToNumber

The ToNumber abstract operation rules are:

• ▪ true becomes 1
• ▪ false becomes 0
• ▪ undefined becomes NaN
• ▪ null becomes 0
• ▪ string: "1" becomes 1, "a" becomes NaN

To coerce an Object value:

1. 1. The ToPrimitive() abstract operation will check if the object has a valueOf() method.
   • ▫ if it exists and it returns a primitive value, that value will be used
   • ▫ if it doesn't exist, but toString() does, its return value will be used
   • ▫ if neither exist or don't return a primitive, a TypeError is thrown
2. 2. The primitive value will be coerced as per the ToNumber rules above.

const objA = {
	valueOf: function(){
		return 1;
	}
};

const objB = {
	toString: function(){
		return "1";
	}
};

const objC = [4,2];
objC.toString = function(){
	return this.join( "" );
};

console.log(Number(objA))
console.log(Number(objB))
console.log(Number(objC))
console.log(Number([]))
console.log(Number(["abc"]))

parseInt

parseInt(..) can be used to get a numeric value out of a string containing non-numeric characters. It parses left-to-right and stops when a non-numeric value is found.

const a = "42px";

console.log(Number(a));
console.log(parseInt(a));

ToBoolean

The ToBoolean abstract operation rules are:

• ▪ undefined becomes false
• ▪ null becomes false
• ▪ +0, -0, and NaN becomes false
• ▪ "" becomes false
• ▪ all other values become true

Boolean(myVar) or !!myVar can be used.

var myVar = 1;

console.log(Boolean(myVar));
console.log(!!myVar);

Implicit Coercion

Expression operations that are implicitly coerced to a boolean:

• ▪ The test expression in an if (..) statement.
• ▪ The test expression (2^nd clause) in a for ( .. ; .. ; .. ) header.
• ▪ The test expression in while (..) and do..while(..) loop.
• ▪ The test expression (1^st clause) in a ? : ternary expression.
• ▪ The left-hand operand to the || and && operators.

Equality

When comparing 2 values for equality, commonly used operators are:

• ▪ loose equality: ==
• ▪ strict equality: ===

Loose allows coercion. Strict doesn't.

const a = 42;
const b = "42";

console.log(a == b);
console.log(a === b);

References

• You Don't Know JS: Types & Grammar - Chapter 4: Coercion