Slice in JavaScript

You are comparing with a single space.

if (str.slice(i, i + 2) === " ") {
//  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
//                          ^^^ single space 

The comparison is only true in one case, at the end of the string with a single space. All other comparisons evaluates to false, because a string with the length of two is never equal to a string with a single character.

It's saying that, for each letter i in the entered text, to the letter i + 2, if they are equal to " ", then they are two spaces.

In other words, it is checking each of the two letter combinations in the text to see if they match two spaces

For example:

"En", "nt", "te", "er", "r ", " s", "so", "om" etc...

So, slice is 'slicing' letters in the range specified in the parameters: str.slice(beginIndex[, endIndex])

Edit: Didn't see the prompt() call, so updated accordingly.

Perhaps try this in order to get a better idea of what's happening:

var str = prompt("Enter some text");
var numChars = str.length;
for (var i = 0; i < numChars; i++) {
    if (str.slice(i, i + 2) === " ") {
        alert("No double spaces!");
        break;
    } else {
        document.write(str.slice(i, i + 2) + "<br>")
    }
}

str.slice(i, i + 2) inside a for loop will parse the string str taking two consecutive characters at a time with indices i and i+1 where i will have all values from 0 to length of str according to the for loop.

Note : In your code snippet by this if(str.slice(i, i + 2) === " ") you are comparing two characters with single character which is white space. So it can never be true.

Have a look at a better example for more clear understanding.

var str1 = "Single space";
var str2 = "Double  space";

var numChars1 = str1.length;
var numChars2 = str2.length;

// Check is str1 has double spaces
for (var i = 0; i < numChars1; i++) {
  if (str1.slice(i, i + 2) === "  ") {
    console.log("Double spaces found in str1");
    break;
  }
}

// Check is str2 has double spaces
for (var i = 0; i < numChars1; i++) {
  if (str2.slice(i, i + 2) === "  ") {
    console.log("Double spaces found in str2");
    break;
  }
}

Learn and find more example about slice on w3schools