有人可以向我解释这个函数的作用吗？

Question

I started learning JavaScript a couple of weeks ago and i always find myself stuck when it comes to for loops.

like in this example, i don't understand what the for loop does , can someone please give me some tips on how to understand this?

let upper = function(strings, ...values) {
    let result = '';
    for (var i = 0; i < strings.length; i++) {
        result += strings[i];
        if (i  < values.length) {
            result += values[i];
        }
    }
    console.log(result);
    return result.toUpperCase();
};

Answer 1

Comments in code will explain what is happening here

 //input is array with string and array with values. //strings, ...values - that is means that first argument will be strings, but all arguments since first will be pushed into array called values let upper = function(strings, ...values) { console.log(strings); // ['a', 'b']; console.log(values); // [1, 2]; let result = ''; // result is an empty string for (var i = 0; i < strings.length; i++) { // looping the array of string result += strings[i]; // result = result + string from strings array at first iteration you will have result equal to 'a' if (i < values.length) { //check do we have number i in values array result += values[i]; // if yes than result = result + string from vaulues. at first iteration you will have result equal to 'a' + 1 that is equal to 'a1' } // end of if } // and so on console.log(result); // 'a1b2' return result.toUpperCase(); // returning result in uppercase }; var res = upper(['a', 'b'], 1, 2); console.log (res); // 'A1B2'

Answer 2

The function adds all the values passed to it as a concatenated sting in all caps

//declares a function in block scope with parameter strings and values which is an array of all other values passed to the function
let upper = function ( strings , ... values ) {
/declares a empty string with block scope called results
let result = '' ;
//a for loop that uses i as a counter
//i is incremented (increased by one) each iteration of the loop until i equals the variable that is passed to the function called strings' length
for ( var i = 0 ; i < strings . length ; i ++) { 
//adds strings[i] to result variable declared above
result += strings [ i ];
//if I is lessthan values length than add values[i] to results
//if I was greater than or equal to values.length than values[i] wouldn't exist and an error would be thrown or Null would be added to results
if ( i < values . length ) { 
   result += values [ i ]; } } 
//outputs result to the console and returns a completely uppercase string from the  result variable to what called the function
console . log ( result ); return result . toUpperCase (); }; 

Answer 3

This is an implementation of a template literal tag. It would be invoked as in

upper`My name is ${name} and I am ${age} years old`

which would evaluate to

MY NAME IS BOB AND I AM 29 YEARS OLD

The template literal mechanism invokes the function named upper , passing as the parameter strings the parts of the template outside the ${} . In this case, it is ["My name is ", " and I am ", " years old"] . As values , it passes a list of the interpolated values coming in between the strings, which in this case is

["Bob", 29]

The code loops through the values in strings . If there is a corresponding (following) interpolated value in values , then it sticks that in. Then it upper cases the entire result and returns it.

For more information on implementing your own template literal tags, see the documentation .

Answer 4

It replaces specific characters of a string at indexes of characters of you pass in as an array of values .

However, one shortcoming - it does not supporting replacing the first character of a passed in string with a corresponding values array value.

values can be either an array or string , but that does not solve the flaw of this function.

Answer 5

 let upper = function(strings, ...values) {  

This is a function declaration, it stores an anonymous function in the variable "upper".

 let result = '';  

This assigns an empty string to "result".

for (var i = 0; i < strings.length; i++) {
result += strings[i];
if (i  < values.length) {
   result += values[i];
}
}

for loop, iterates over the strings passed to the anonymous function and stores the results in "result". If there are other parameters, these are stored in "result" as well.

console.log(result);
return result.toUpperCase();
};

Here it calls console.log with result as argument, then returns the result value, after running the native toUpperCase method on result.

You cannot have the function like that, it will not run, the arguments shouldn't be "...values". But that is what it does, takes a value and returns a string as uppercase lettered string.