Dynamic typing

JavaScriptJunior~1 min read

What is typing

Typing is the way a programming language manages data types (numbers, strings, objects, and so on).

Static vs Dynamic typing

Characteristic	Static typing	Dynamic typing
When the type is determined	At compile time	At run time
Can the variable's type change	No	Yes
Example languages	TypeScript, Java, C#, C++	JavaScript, Python, PHP
Type checking	Before the program runs	During execution
Error type on mismatch	Compilation error	Runtime error

Example: JavaScript (dynamically typed)

javascript

let value = 42;       // number
console.log(typeof value); // "number"

value = "Hello";      // now a string!
console.log(typeof value); // "string"

value = true;         // now a boolean value
console.log(typeof value); // "boolean"

Here, the variable's type can change at runtime, and this is normal for JS.

Comparison example: TypeScript (statically typed)

javascript

let value: number = 42;
value = "Hello"; // Error: type "string" is not assignable to type "number"

In TypeScript (or Java), the type is specified once and cannot change.

Consequences of dynamic typing

Pros:

Faster code writing, fewer "noisy" annotations.
Flexibility: variables can be reused with different types.

Cons:

Errors only show up at runtime.
Harder to debug large projects.
Unexpected type conversions are possible:

javascript

console.log('5' - 2); // 3   (string → number)
console.log('5' + 2); // "52" (number → string)

Summary

JavaScript is a dynamically and weakly typed language, because:

the variable's type is determined at runtime,

and can change at any moment,

while the language automatically converts types when possible.

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