JS: || outside of if statement

Question

I'm trying to understand this code (check whether a string can be re-arranged to a palindrome) :

function canRearrangeToPalindrome(str){
    var letterCounts = {};
    var letter;
    var palindromeSum = 0;
    for (var i = 0; i < str.length; i++) {
        letter = str[i];
        letterCounts[letter] = letterCounts[letter] || 0;
        letterCounts[letter]++;
    }
    for (var letterCount in letterCounts) {
        palindromeSum += letterCounts[letterCount] % 2;
    }

    return palindromeSum < 2;
}

Could you explain line letterCounts[letter] = letterCounts[letter] || 0; It's outside of if statement, how can we use || ? Thank you!

if there isn't a truthy value in letterCounts[letter] it will set it to 0 — Cruiser, Oct 16 '17 at 15:34
Here is a test script that you can run in the console: `test = [0, 1]; result = test[2] || 'This index does not exist.';` On the first line, we define `test` as an array with 2 indices. One the second line, we request the 3rd index of `test`, which doesn't exist. Since there is no value for `test[2]`, the operation returns 'This index does not exist'. — Philip Raath, Oct 16 '17 at 15:42

score 0 · Accepted Answer · answered Oct 16 '17 at 15:34

0

If the index at letter in letterCounts has a falsy value then set the value at that index to 0

Here is the MDN documentation for Falsy: https://developer.mozilla.org/en-US/docs/Glossary/Falsy

answered Oct 16 '17 at 15:34

Tom O.

5,730
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score 0 · Answer 2 · answered Oct 16 '17 at 15:35

0

If letterCounts[letter] has a value it will assign that value otherwise it will assign 0.

answered Oct 16 '17 at 15:35

Kenji Mukai

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JS: || outside of if statement

2 Answers2