TypeScript: no error when using `+` operator on incompatible types

Bug Report

today i was shocked to learn that something like 1 + "2", one of the most common examples of why javascript is unsafe, doesn’t cause an error in typescript.

i get that it technically works at runtime, but so does 1 + undefined which correctly causes an error

related: #30239

🔎 Search Terms

concatenate incompatible types operator

(i’d be very surprised if this hasn’t been raised before, but i couldn’t find an existing issue no matter what search terms i used)

🕗 Version & Regression Information

4.7.0-dev.20220504

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.7.0-dev.20220504#code/IwAg1CBEBMlA

💻 Code

1 + "2"

🙁 Actual behavior

no error

🙂 Expected behavior

error when using + operator on incompatible types

About this issue

Original URL
State: closed
Created 2 years ago
Reactions: 4
Comments: 20 (11 by maintainers)

Most upvoted comments

Duplicate of https://github.com/microsoft/TypeScript/issues/16895.

fatcerberus on May 5, 2022

kotlin

1 + "2" // error
"1" + 2 // no error

Kotlin moment

KotlinIsland on May 5, 2022

imo you would typically use string interpolation for cases like that. the languages i’m used to seem to make the same assumption.

python (mypy)

1 + "2" # error
f"{1}2" # no error

dart

void main() {
  1 + "2"; // error
  "${1}2"; // no error
}

kotlin

fun main() {
    1 + "2" // error
    "${1}2" // no error
}

DetachHead on May 5, 2022