Lesson 03 of 55
8 mins readPython Type Inference & Type Widening
In Plain English
TypeScript can automatically infer types without explicit annotations. Variables declared with `let` infer widened types (like `string`), while `const` infers narrow literal types.
Deep Dive: How It Works
Type Inference: TypeScript automatically determines the type when a variable is initialized (`let x = 10` is inferred as `number`).
Const Widening: `const mode = "dark"` infers the literal type `"dark"`; `let mode = "dark"` infers `string`.
Contextual Typing: Parameter types are inferred from caller context (e.g. inside `array.map(x => ...)`).
Core Rules to Remember
Omit Redundant Annotations: Writing `let x = 10` is cleaner than `let x: number = 10`.
Contextual Inferences: Higher-order callbacks infer argument types from the parent array/function.
Live Interactive Example
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Python 3.12
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Output Console
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How it works: TypeScript automatically inferred the array type and the map callback argument type.
Your Turn: Micro Challenge
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Micro Exercise
Use Contextual Inference in Array Filter
- Declare `const scores = [65, 88, 92, 45, 99];`
- Filter scores `>= 80` using `scores.filter(s => s >= 80)` without explicit type annotations.
- Log `"Honor scores: " + passing.join(", ")`.
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Sandbox Output
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