CS 349 TypeScript Intro

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Truthy ​

JavaScript automatically converts values to booleans in conditional contexts like if statements or && operations. This implicit conversion follows specific rules that can sometimes be surprising, especially for developers coming from other languages. Using strict equality (===) instead of loose equality (==) helps limit these surprises by preventing automatic type conversion.

Truthy and Falsy ​

Values are automatically converted to true or false based on these rules:

These falsy values, all convert to false:

js
false       // boolean false
0           // number zero
""          // empty string
null        // null
undefined   // undefined
NaN         // Not a Number

Everything else is truthy, here are some examples:

js
if ("hello") {     // true - non-empty string
    console.log("truthy!");
}

if (42) {          // true - any non-zero number
    console.log("truthy!");
}

if ([]) {          // true - empty array is truthy!
    console.log("truthy!");
}

if ({}) {          // true - empty object is truthy!
    console.log("truthy!");
}

⚠️ Be careful! Empty arrays and objects are truthy, which can be surprising since in Python an empty list [] is false. In TypeScript, you'll usually check specific conditions instead of relying on truthy/falsy conversion.

Equality vs Strict Equality ​

The loose equality operator (==) also performs type conversion, leading to some surprising results:

js
0 == ""        // true! (both are falsey)
1 == "1"       // true! (string converted to number)

Strict equality (===) doesn't convert types

js
0 === ""       // false (number vs string)
1 === "1"      // false (number vs string)

🛑 Always use strict equality (===) to avoid unexpected type conversion.

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