Re-understand JavaScript

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In this post, we will be looking at a few interesting (but could be challenging) JavaScript questions.

Most of them are actually testing the so-called “down-side” of JavaScript. You certainly should not write such code in a real-world codebase. As I have repeated many times, code should be clear, precise and concise, in that order of importantce. Nevertheless, they are indeed good questions to test your competency in JavaScript.

9 or 10?

You are given a function called magic_length, which is defined as follows:

1
2
3
function magic_length(input) {
    return input.length == 10 && input == ",,,,,,,,,";
}

Please give one possible value of input such that magic_length(input) will return true. Notice: input should be of basic data type provided by built-in libraries.

Solution

One possible value of input is [undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined] (an array of 10 items, each of which is undefined).

Analysis

Let’s first think about what shall be the data type of input. input must be of such a data type that it comes with the length attribute. Therefore, input can only be of type string or array.

We also notice that the statement on Line 2 is using the abstract equality comparison (==) rather than strict equality comparison (===). This “double equal” will only check for loose equality and will perform a type conversion when the two items are not of the same type. See here for official documentation.

  • If input is a string:
    • Let’s look at the 2nd comparision first. When comparing two strings, no type conversion is done. Thus, input has to be ,,,,,,,,,. Then, it is impossible for its length to be 10.
  • If input is an array:
    • When comparing an array with a string, input will be converted to string by ToPrimitive(input). Inferring from the first part, this array would have 10 items. Thus, now the problem becomes: given an array with 10 items, what would be the value when applying ToPrimitive on it would result in ,,,,,,,,,.

According to ECMAScript 2015 Language Specification, the toString method of array will be called. toString joins the array and returns one string containing each array element separated by commas. Hence, ,,,,,,,,, actually comes from the commas used to separate the elements. Since there are 10 items in input, there shall be 9 commas to separate them.

Further, that means when 10 items in input are converted to string format, they become empty string. A simple guess would be either null or undefined. When null is converted to string format, it will become "null"; when undefined is converted to string format, it will become "".

Therefore, we can conclude that input is an array containing 10 undefined inside.