Functions
JavaScript Lesson 10 2:19 English narration · English + 中文 subtitles burned in
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Transcript
A function is a job with a name.
函数就是一件有名字的工作。
You write the word function, which tells JavaScript you are defining something rather than running it — nothing in the body happens until somebody calls it.
你先写 function 这个词， 它告诉 JavaScript 你是在定义一个东西，而不是在运行它—— 在有人调用之前，函数体里什么都不会发生。
Then a name, chosen the way you choose a variable name.
接着是名字，取名的规矩和取变量名一样。
Then, in the brackets, the parameters: names for the values the caller will pass in.
然后是括号里的形参：调用者传进来的那些值的名字。
And the body goes in curly braces underneath.
函数体写在下面的花括号里。
Using it looks like this.
用起来是这样的。
You hand it a value — four — which arrives inside as n.
你递给它一个值——4——它在里面就成了 n。
The body runs.
函数体开始执行。
Then return hands one back: eight.
然后 return 交还一个值：8。
And the last step is the one beginners skip: store what comes back, or use it straight away, because the value arrives at the place where the call was written and nowhere else.
最后一步正是初学者容易跳过的：把返回的值存起来，或者立刻用掉， 因为这个值只会到达写下这次调用的地方，别的地方都不会有。
These two look almost identical and behave completely differently.
这两段看起来几乎一样，行为却完全不同。
The first shows it and keeps nothing: the function ends without producing a value, so x is undefined, and adding one to undefined gives NaN — not a number.
第一个只是把结果显示出来，什么也没留下：函数结束时没有产生值， 所以 x 是 undefined，再给 undefined 加 1 得到 NaN——不是一个数字。
The second hands it back: x is eight, and the next line logs nine.
第二个把结果交还回来：x 是 8，下一行打印出 9。
The test is simple — if you cannot store the answer in a variable, the function returned nothing.
判断办法很简单——如果你没法把答案存进一个变量，那这个函数就什么都没返回。
Now the lesson's third task, and it is deliberately the other kind.
现在做课程里的第三道题，它故意是另一种函数。
Greet takes a name and logs a greeting — and it has no return at all.
greet 接收一个名字并打印一句问候——而且它完全没有 return。
That is correct here, because its job IS the logging; there is no answer for a caller to catch.
在这里这是对的，因为它的工作就是打印；没有什么答案需要调用者去接。
One thing to notice: defining it does nothing on its own.
有一点要注意：光是定义它，什么也不会发生。
You have to call it on the line below, with a real name in the brackets, before anything appears.
你必须在下面那一行调用它，括号里给一个真实的名字，才会有东西出现。
Four things to take with you.
带走四点。
One: function, a name, brackets, braces.
第一：function、名字、括号、花括号。
Two: return hands one value back to whoever called it.
第二：return 把一个值交还给调用它的地方。
Three: console dot log shows a value to a human — only return gives a value to the rest of your program.
第三：console.log 是给人看的——只有 return 才能把值交给你程序的其余部分。
Four: defining is not running, so you must call it.
第四：定义不等于运行，所以你必须去调用它。
Now do the three tasks; two return, and the last one does not.
现在去做那三道题；两题要返回，最后一题不用。