الدرس 07 · Basics

Functions

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A function is a named list of steps you write once and run whenever you call it. Follow a seven-line script that prices two orders with one function, total(price, count): each call hands in arguments, gets its own call frame of local variables, and return sends one value back to the line that called. console.log then prints 22.

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01In function total(price, count) { ... }, what are price and count?

Parameters are the names in the function's brackets. A call such as total(3, 4) fills them in order: price = 3, count = 4.

02When JavaScript reaches the function declaration on line 1, what happens to lines 2 and 3?

Declaring a function stores it under its name. The body runs only when a line calls the function, once per call.

03What does return cost; do inside total?

return ends the call. total(3, 4) on line 5 is replaced by the value 12, which const tea then keeps.

04After line 5 has run, a new line console.log(cost); is added below it, outside the function. What happens?

cost is a local variable. It existed only in that call's frame, which was cleared when the call returned.

05What does a function send back if it has no return statement?

A function that ends without return, or with a bare return;, gives back undefined.

المصطلحات الأساسية

المسرد

Function: A named list of steps, written once, that runs each time it is called.
Function declaration: A statement such as function total(price, count) { ... } that creates a function and binds it to a name. It is hoisted: the name works from the top of the script.
Parameter: A name in a function's brackets, such as price, that each call fills in with a value.
Argument: A value passed in a call, such as 3 in total(3, 4).
Call: Running a function by writing its name followed by brackets and arguments, as in total(3, 4).
return: A statement that ends a call and sends a value back to the place the call was made.
Local variable: A variable that exists only inside one call of a function, such as price, count or cost.
Call frame: The scratch space a call gets for its local variables; the ECMAScript specification calls it an execution context. It is removed when the call returns.

تفاصيل إضافية

الملاحظات والمصادر

The drawing shows each call’s local variables on a tray, the call frame. The ECMAScript specification calls it an execution context: every call gets a new one with its own record of local names, it goes on the call stack, and it is removed when the call returns.
A function declaration is hoisted: JavaScript creates the function and binds the name total before the first line runs, so a call could even come above the declaration. const tea and const cake get their values only when their lines run.
A function that ends without a return statement, or with a bare return;, sends back the value undefined.
Here the arguments are numbers, so the frame gets its own copies of them: changing price inside total would not change anything outside. For an object, what is copied is a reference to the same object.
JavaScript has other ways to write a function. const total = (price, count) => price * count; is an arrow function that gives the same answers and is called the same way.
All seven lines are real: paste them into the browser console, or save them as total.js and run node total.js. Both print 22.

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