Can you explain the distinction between String[] and [String] in typescript? Which option would be more advantageous to use?
Can you explain the distinction between String[] and [String] in typescript? Which option would be more advantageous to use?
They are actually quite different!
New Information: Ever since TypeScript 2.7 was introduced, the distinction has become less clear (more details below):
string[] indicates an array with elements of type string (it can have any number of elements, including none).[string] signifies an array with at least one element, and that first element must be a string
[type] syntax can be expanded, such as [type1,type2,type3,typeN], which then requires the array to have at least N elements, with the first N being specified types, while the subsequent types are a union of those types.Below are some examples to clarify this concept:
const testA: string[] = []; // correct
const testB: [string] = []; // Error, needs a string at index 0
const testC: [string, number] = ["a", 0]; // correct
const testC1 = testC[0]; // testC1 is recognized as a string
const testC2 = testC[1]; // testC2 is identified as a number
const testD: [string, number] = ["a", 0, "1"]; // acceptable before 2.7, invalid after
const testD1 = testD[2]; // testD1 is interpreted as string | number
const testE: [string, number] = ["a", 0, 1]; // acceptable before 2.7, problematic after
const testE1 = testE[2]; // testE1 is inferred to be string | number
const testF: [string, number] = ["a", 0, null]; // Error, null doesn't match type string|number
Changes in TypeScript 2.7
Now, the size is fixed by default. To allow [string] to have more than one entry, you need to specify it using a rather cumbersome method:
interface MinimumStrTuple extends Array<number | string> {
0: string;
}
Let's go back to the basics - what exactly is TypeScript? TypeScript serves as a typed superset of JavaScript that ultimately compiles down to javascript.
In contrast, when writing code in JavaScript without types, you may run into issues like this:
// It's acceptable to have code like
let str = 'sfafsa' ;
str = true ;
This is where TypeScript comes in handy by allowing you to write code with specified types and flagging errors before compilation if you try assigning a value that does not match the variable type. For instance, if you experiment with different ways to define an array.
They end up being identical, why? Because TypeScript will render them the same way. For instance:
let str: String[] = ['ge', 'gege', 'egeg'];
let str2: [String] = ['ge', 'gege', 'egeg'];
Both will be compiled to:
var str = ['ge', 'gege', 'egeg'];
var str2 = ['ge', 'gege', 'egeg'];
Therefore, they compile to the same result, so feel free to choose whichever method works best for you. I suggest sticking to one convention throughout your code base. By the way, I personally prefer using syntax like let str: String[]=[];
Note: If you opt for the syntax [String], it implies that your array must contain at least one String >> therefore, an empty array will trigger an error.
A big thank you to @Lusito for making sure the answer was thorough.
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