Type Annotations and Type Checking
12345678// Variable declared with type annotation let age: number = 30; // Variable declared without type annotation (TypeScript infers the type) let userName = "Alice"; // Type error: assigning a string to a number variable // age = "thirty"; // Error: Type 'string' is not assignable to type 'number'
TypeScript allows you to declare the type of a variable using a type annotation, as shown with let age: number = 30;. Here, you explicitly tell TypeScript that age must always hold a number. When you declare a variable without a type annotation, like let userName = "Alice";, TypeScript uses type inference to determine that name is a string based on the assigned value.
TypeScript performs type checking at compile time, meaning it analyzes your code before it runs. If you try to assign a value of the wrong type—such as assigning a string to the age variable—TypeScript immediately reports an error and prevents the code from compiling. This compile-time type checking helps you catch mistakes early and write safer, more predictable code.
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Can you explain more about type inference in TypeScript?
What happens if I don't specify a type and don't assign a value?
Are there other ways to declare variable types in TypeScript?
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12345678// Variable declared with type annotation let age: number = 30; // Variable declared without type annotation (TypeScript infers the type) let userName = "Alice"; // Type error: assigning a string to a number variable // age = "thirty"; // Error: Type 'string' is not assignable to type 'number'
TypeScript allows you to declare the type of a variable using a type annotation, as shown with let age: number = 30;. Here, you explicitly tell TypeScript that age must always hold a number. When you declare a variable without a type annotation, like let userName = "Alice";, TypeScript uses type inference to determine that name is a string based on the assigned value.
TypeScript performs type checking at compile time, meaning it analyzes your code before it runs. If you try to assign a value of the wrong type—such as assigning a string to the age variable—TypeScript immediately reports an error and prevents the code from compiling. This compile-time type checking helps you catch mistakes early and write safer, more predictable code.
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