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Foundations
Let and var Type annotations Type inference Numeric types Strings and interpolation Tuples Optionals Nil coalescing
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Modern Swift
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Swift / Foundations

Let and var

Learn let and var in Swift through a focused explanation, a dedicated topic-specific example, an engineering boundary, and a mastery check.

10 min read • Beginner

let and var is an atomic Swift topic inside Foundations. The example uses the named construct directly so you can see its syntax, data flow, and observable behavior.

Read the code by locating the exact feature named let and var, then trace the value before and after that feature runs. This lesson is intentionally isolated from neighboring topics: changing identifiers alone would not reproduce the same behavior because the construct itself is different.

Topic-specific example

swift-0001-62bfb439b7.swift
swift Try It Live 
let id = 7
var quantity = 1
quantity += 2
print(id, quantity)
Edge case / engineering note

For let and var, test the boundary that makes the feature meaningful: invalid input, an empty value, a missing branch, lifecycle/cleanup, or a type/runtime constraint. Do not infer correctness only from the happy path.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

Let and var Mastery Check →
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Type annotations