if is not a function but is a keyword. There is no JavaScript implementation of it.

The compiler/transpiler understands the if statement just like it understands other tokens from the var, const, and let to do {} while(); and for. All of these are keywords with special meaning known to the JavaScript interpreter.

You would want to look the implementation of the interpreter/compiler and runtime implementation to understand how it handles the various control keywords such as if. Alternatively you can look at the specs of the language.

Edit:

It is worth noting that JavaScript is ECMAScript. There are ECMAScript standards and JavaScript implements these. So concepts such as truthiness and falsiness (whether a value is truthy of falsey) is in the ECMAScript standards.

Particularly, you can check section 13.6.7 that speaks to the semantics of the if statement: https://262.ecma-international.org/11.0/#sec-if-statement-runtime-semantics-evaluation

From there, one can find what it means for something to be truthy/falsey, which is ToBoolean, as defined in the specs (not a JavaScript function):