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Boolean

The type of boolean in TypeScript is boolean. It also have boolean literal types true and false

Predicate functions should return boolean.

// bad
function hasValue(value: any) {
  return value
}

// good
function hasValue(value: any) {
  return value !== undefined
}

Why?

Relying on implicit conversion is dangerous. Always be explicit.

hasValue(0) ? true : false // false
hasValue(false) ? true : false // false
hasValue('') ? true : false // false
hasValue(Symbol()) ? true : false // false
hasValue(Infinity) ? true : false // false
// but
new Boolean(Infinity) // true !!

When converting value to boolean, you should use double not (!!) operator.

const value = false
// bad
const b = new Boolean(value)
if (b) { /* executed! */ }

// so so
const c = Boolean(value)
if (c) { /* not executed */ }

// good
const d = !!value
if (d) { /* not executed */ }

Why?

In 99.99999% of the time, you do not even know the existence of the boolean object wrapper Boolean. It is different then the boolean you use days in days out.

So don't confuse yourself and your reader by mentioning it in your code when not necessary.

References