In programming, a contextual type refers to a type that is inferred based on the context in which an expression is used. This means that the type of a variable or an expression can be determined by the surrounding code.
Let’s consider an example to understand this better. Imagine we have the following code snippet:
const x = 5; // Here the contextual type of 'x' is inferred as number
const y = "Hello"; // Here the contextual type of 'y' is inferred as string
const z = [1, 2, 3, 4]; // Here the contextual type of 'z' is inferred as number[]
In this example, the types of variables ‘x’, ‘y’, and ‘z’ are inferred by the assignment values. The contextual type of ‘x’ is inferred as ‘number’ because the value assigned is a numeric literal. Similarly, the contextual type of ‘y’ is inferred as ‘string’ because the assigned value is a string literal. Finally, the contextual type of ‘z’ is inferred as an array of numbers because the assigned value is an array containing numeric literals.
In many programming languages with type inference, the contextual type can save developers from explicitly specifying types, making the code more concise and readable. Additionally, it allows the flexibility of changing the type of a variable if needed, without modifying every occurrence of that variable in the code.