JavaScript syntax
part 10/31 · 107.4 KB total
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Douglas Crockford advocates the terms "truthy" and "falsy" to describe how values of various types behave when evaluated in a logical context, especially in regard to edge cases.cite-ref-14[14] The binary logical operators returned a Boolean value in early versions of JavaScript, but now they return one of the operands instead. The left–operand is returned, if it can be evaluated as : false, in the case of conjunction: (a && b), or true, in the case of disjunction: (a || b); otherwise the right–operand is returned. Automatic type coercion by the comparison operators may differ for cases of mixed Boolean and number-compatible operands (including strings that can be evaluated as a number, or objects that can be evaluated as such a string), because the Boolean operand will be compared as a numeric value. This may be unexpected. An expression can be explicitly cast to a Boolean primitive by doubling the logical negation operator: (!!), using the Boolean() function, or using the conditional operator: (c ? t : f).
// Automatic type coercion
console.log(true == 2 ); // false... true → 1 !== 2 ← 2
console.log(false == 2 ); // false... false → 0 !== 2 ← 2
console.log(true == 1 ); // true.... true → 1 === 1 ← 1
console.log(false == 0 ); // true.... false → 0 === 0 ← 0
console.log(true == "2"); // false... true → 1 !== 2 ← "2"
console.log(false == "2"); // false... false → 0 !== 2 ← "2"
console.log(true == "1"); // true.... true → 1 === 1 ← "1"
console.log(false == "0"); // true.... false → 0 === 0 ← "0"
console.log(false == "" ); // true.... false → 0 === 0 ← ""
console.log(false == NaN); // false... false → 0 !== NaN
console.log(NaN == NaN); // false...... NaN is not equivalent to anything, including NaN.
// Type checked comparison (no conversion of types and values)
console.log(true === 1); // false...... data types do not match
// Explicit type coercion
console.log(true === !!2); // true.... data types and values match
console.log(true === !!0); // false... data types match, but values differ
console.log( 1 ? true : false); // true.... only ±0 and NaN are "falsy" numbers
console.log("0" ? true : false); // true.... only the empty string is "falsy"
console.log(Boolean({})); // true.... all objects are "truthy"
The new operator can be used to create an object wrapper for a Boolean primitive. However, the typeof operator does not return boolean for the object wrapper, it returns object. Because all objects evaluate as true, a method such as .valueOf(), or .toString(), must be used to retrieve the wrapped value. For explicit coercion to the Boolean type, Mozilla recommends that the Boolean() function (without new) be used in preference to the Boolean object.
const b = new Boolean(false); // Object false {}
const t = Boolean(b); // Boolean true
const f = Boolean(b.valueOf()); // Boolean false
let n = new Boolean(b); // Not recommended
n = new Boolean(b.valueOf()); // Preferred
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