What does this `…${…}…` code in the node docs mean

问题:

This question already has an answer here:

• What is the usage of the backtick symbol (`) in JavaScript? 5 answers

I am to trying to learn Express library and Node.js one step at a time. First I am looking at is the specifics of the Node reqiure(moduleName) function.

I took a look at the documentation for this, and found some weird code in the example documentation:

const circle = require('./circle.js');
console.log( `The area of a circle of radius 4 is ${circle.area(4)}`);

More specifically the ${circle.area(4)} bit.

From what I understand the $ in JavaScript is just like any other variable. When we are using it on client side web development it is used as a delegate for the document function (I think). What is it assigned to when using node?

On top of that, what does this syntax mean? ${circle.area(4)}

If $ is just a reference to some function someFunction(), wouldn't it be equivalent to this someFunction(){cirle.area(4)}. I am not seeing how that could be valid syntax.

Also, why wouldn't they just directly call the circle.area() function directly anyways?

回答1:

This:

`The area of a circle of radius 4 is ${circle.area(4)}`

is an example of ES2015 template strings.

It interpolates whatever circle.area(4) represents directly into the string. If you're curious about this or other ES2015 features, I recommend checking out Babel and playing around in the REPL.

Here's a very simple example to get you started.

You can see this ES2015 code:

const foo = 'some text';
console.log(`${foo} is interpolated.`);

is transpiled to its ES5 equivalent - a simple + concatenation:

var foo = 'some text';
console.log(foo + ' is interpolated.');