Take a look at the code sample below.
let array = [,,undefined,1,2,,]; console.log(array);
Elision: omission of a passage in a book, speech, or film.https://www.google.com/search?q=Elision+definition
Making Sparse Arrays
1. Using the Array Constructor
The simplest way is to use the Array Constructor with a count.
let sparseArray = new Array(10); console.log(sparseArray); // (10) [empty × 10]
2. Assigning a value to an index after length
let a = ; a = 0; // assign value to index after length console.log(a); // [empty x 5, 0]
3. Changing the length property
let b = ; b.length = 5; // change length property console.log(b); // [empty x 5]
4. Deleting elements from an array
Note: deleting array elements does not alter the array length. Rather it puts holes in those locations. Again, holes are not same as undefined, they are empty slots.
let c = [1,2,3,4]; delete c, delete c, delete c; // delete console.log(c); // [empty x 3, 4]
You can read more about the comma operator here.
2. Using the Array literal
let d = [,,,,]; // empty slots console.log(d); // [empty x 4], last comma doesn't count
Undefined vs Empty?
let weird = new Array(2); weird = undefined; console.log(weird); // [undefined, empty] console.log(weird); // undefined console.log(weird); // undefined weird === weird; // true weird === undefined; // true
How do you know if an array slot is empty or undefined? Obviously comparing to undefined does not work. This is where the in operator is useful.
Seeing empty slots: The in Operator
The in operator is an object operator that checks if a property exists in an object or its prototype chain. Since arrays are objects, this is a foolproof way to identify holes. Empty slots have no defined index values at their locations because the property does not exist.
let sparseArray = new Array(10); sparseArray.length // 10 0 in sparseArray // false 9 in sparseArray // false sparseArray = undefined; 0 in sparseArray // true
This shows that the 0th slot is now filled with undefined and not empty.
Why do arrays need to be sparse?
Performance. A huge sparse array e.g. new Array(9999999) contains nothing until elements are inserted.
The V8 engine optimizes for performance by distinguishing between dense and sparse arrays. See this great article on HOLEY and PACKED element kinds: https://v8.dev/blog/elements-kinds.Tweet
Large Sparse Array
let size = 500000000; console.time(); let timed = new Array(size); console.timeEnd(); // ~ 0.03 ms
Large Dense Array
Warning: this snippet will crash your browser.
let size = 500000000; console.time(); let timed = new Array(size)fill(0); console.timeEnd();
- Sparse arrays contain holes
- Dense arrays contain contiguous elements (even if they are undefined).
The ECMAScript standard leaves implementation up to browser vendors to choose how to optimize.
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