Javascript Coercion and Operator Between Non-boolean Values

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Definition

Converting a value from one type to another is often called “type casting,” when done explicitly, and “coercion” when done implicitly (forced by the rules of how a value is used).
Another way these terms are often distinguished is as follows: “type casting” (or “type conversion”) occur in statically typed languages at compile time, while “type coercion” is a runtime conversion for dynamically typed languages.

var a = 42;

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

var c = String( a );	// type casting

Coercion to false

From that table, we get the following as the so-called “falsy” values list:

• undefined
• null
• false
• +0, -0, and NaN
• “”

These value will be coerce to false if you force a boolean coercion on it.

Usage

Using Logical Operators with Non-Boolean Values

The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

"foo" && "bar"; // "bar"
"bar" && "foo"; // "foo"
"foo" && "";    // ""
""    && "foo"; // ""

"foo" || "bar"; // "foo"
"bar" || "foo"; // "bar"
"foo" || "";    // "foo"
""    || "foo"; // "foo"

Both && and || result in the value of (exactly) one of their operands:

• A && B returns the value A if A can be coerced into false; otherwise, it returns B.
• A || B returns the value A if A can be coerced into true; otherwise, it returns B.

Reference:

1. https://mariusschulz.com/blog/the-and-and-or-operators-in-javascript
2. https://github.com/getify/You-Dont-Know-JS/blob/master/types%20&%20grammar/ch4.md#operators–and-