Mastering JavaScript

ECMAScript 6
The final variation of the template strings is called tagged template string. The idea
is to modify the template string using a function. Consider the following example:
function emmy(key, ...values){
console.log(key);
console.log(values);
}
let category="Best Movie";
let movie="Adventures in ES6";
emmy`And the award for ${category} goes to ${movie}`;
//["And the award for "," goes to ",""]
//["Best Movie","Adventures in ES6"]
The strangest part is when we call the emmy function with the template literal. It's not
a traditional function call syntax. We are not writing emmy(); we are just tagging the
literal with the function. When this function is called, the first argument is an array
of all the plain strings (the string between interpolated expressions). The second
argument is the array where all the interpolated expressions are evaluated and
stored.
Now what this means is that the tag function can actually change the resulting
template tag:
function priceFilter(s, ...v){
//Bump up discount
return s[0]+ (v[0] + 5);
}
let default_discount = 20;
let greeting = priceFilter `Your purchase has a discount of
${default_discount} percent`;
console.log(greeting); //Your purchase has a discount of 25
As you can see, we modified the value of the discount in the tag function and
returned the modified values.
Maps and Sets
ES6 introduces four new data structures: Map, WeakMap, Set, and WeakSet.
We discussed earlier that objects are the usual way of creating key-value pairs in
JavaScript. The disadvantage of objects is that you cannot use non-string values as
keys. The following snippets demonstrate how Maps are created in ES6:
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