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GeezerVison is showing github.com — Let and const support by mhegazy · Pull Request #904 · microsoft/TypeScript

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Mohamed Hegazy (mhegazy) commented Oct 16, 2014

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This change adds support for ES6-style block-scoped variable declarations. these are only available when targeting ES6. the change also adds a new target for ES6.

There are two variants, let and const. Generaly let and const behave like a var declarations with the following exceptions:

Both let and const can not be re-declared:

let a = 0;
a = 1;
let a; // Error: redeclaration

It is an error to re-declare a block-scoped variable as 'var' in the same scope:

let a = 0;
{
     var a; // Error: redeclaration, as var is hoisted to the top of the scope, and re-declares a
}

Let and const can not be declared in a non-guarded statements

if (true) {
    let a = 0; // OK
}

if (false) 
   let b = 0; // Error

for (let i = 0; i< 10; i++)  // OK
    console.log(i);

Let and const declarations are block scoped, and not hoisted to the top of the function like other JS declarations:

let a = 0;
{
    let a = "local";
    console.log(a) // local
}
console.log(a); // 0

Let and const can not be used before they are defined, to avoid the lexical dead zone of vars.

v = 2; // ok
var v;

a = 2;
let a; // Error, used before initialization

Const declarations must have an initializer, unless in ambient context
It is an error to write to a Const

const c = 0;
console.log(c); // OK: 0

c = 2; // Error
c++; // Error

{
    const c2 = 0;
    var c2 = 0; // not a redeclaration, as the var is hoisted out, but still a write to c2
}

Let and const can not be exported, only vars are allowed to.

What is left:

Block emit if any of the let/const errors are reported, these may be syntactic, binding, or semantic errors
Wire in Test262 for parser verification to ensure we are ES6 complaint

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Mohamed Hegazy (mhegazy) added 15 commits

October 11, 2014 12:52


          Add es6 target

873c1df


          Add basic parsing support for let and const

778f101


          Disallow let and const declarations outside blocks

979d45e


          Fix line endings

6f6f4af


          Add binder support for block scoped variable declarations

cf89f5c


          Do not allow use of block-scoped variable before its definition

1dde985


          Ensure duplicate let/const declarations accross files are reported

318575c


          Report duplicate identifier errors on all locations for merged declar…

cffc62a

…ations to align with local declarations


          Flag assignments to a const

f5c2740


          Flag const declarations shodowed by var redeclarations

82f5fb4


          Allow const in for statements

3e45601


          Do not allow let and const declarations to be exported from a module

03a100d


          Fix emitting for const in for loops


          Merge branch 'master' into letAndConst

e15f4e6

Conflicts:
	src/compiler/types.ts


          Add language service support for const

60bb37b

CyrusNajmabadi reviewed

Oct 16, 2014

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src/compiler/parser.ts

Cyrus Najmabadi (CyrusNajmabadi) Oct 16, 2014

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i thought we would not allow 'let' here as we're not in a block scope. or do you check for that later?

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Mohamed Hegazy (mhegazy) Oct 17, 2014

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i think you are correct. the ES6 spec makes it clear that lexical scopes do not include switch, try or finally blocks, section 8.1:

A Lexical Environment is a specification type used to define the association of Identifiers to specific variables and functions based upon the lexical nesting structure of ECMAScript code. A Lexical Environment consists of an Environment Record and a possibly null reference to an outer Lexical Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement and a new Lexical Environment is created each time such code is evaluated.

I will follow up to see if this is intentional or just an error in the spec and update this accordingly.

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Jason Freeman (JsonFreeman) Oct 17, 2014

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They say "such as", which I normally interpret as "including but not limited to".

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Mohamed Hegazy (mhegazy) Oct 17, 2014

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yeah. checked with Brian Terlson (@bterlson) and he confirmed that that is not the intention. so keeping Switch, try and finally blocks as valid lexical scopes

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Anders Hejlsberg (ahejlsberg) commented Oct 17, 2014

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Overall looks great!

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Mohamed Hegazy (mhegazy) added 10 commits

October 17, 2014 09:28


          Fix search for shadowed const declarations by a var declarations to s…

fd469d6

…earch for any variable instead of only a blockScoped one to ensure we are not picking it up from a wrong scope.


          Respond to code review comments

4ef68b9


          Allow const and let declarations to be exported in modules. Also ensu…

a5a6c6f

…re that const module elements are not used as references.


          Treat blockScoped variable declarations as a separate category when i…

0a59cdd

…t comes to symbol flags, instead of compining BlockScoped and Variable


          Merge branch 'master' into letAndConst

0e7d8b6


          Update error messages

dd5c89d


          Simplify the binder logic for managing blockScopeContainer

91f4098


          allow let and const declarations in module bodies under labels

d5fe43b


          Create a new flag for diagnostics 'isEarly' and disable emit if this …

dd7ca69

…flag is set. Set the flag by default on all let and const errors to ensure we are not emitting invalid JS code.


          respond to code review comments

373dc76

Mohamed Hegazy (mhegazy) added 5 commits

October 23, 2014 15:01


          Merge branch 'master' into letAndConst

9353c11

Conflicts:
	src/compiler/types.ts


          Merge branch 'master' into letAndConst

d1858d0


          Ensure let and const declarations in labels are parsed correctelly

e4a2084


          Only check for collisions with variabels and not properties

67c78a2


          Merge branch 'master' into letAndConst

51e101c

Mohamed Hegazy (mhegazy) added a commit that referenced this pull request

Oct 24, 2014


          Merge pull request #904 from Microsoft/letAndConst

290e43b

Let and const support

Mohamed Hegazy (mhegazy) merged commit 290e43b into master

Oct 24, 2014

Mohamed Hegazy (mhegazy) deleted the letAndConst branch

October 24, 2014 18:26

This was referenced Oct 25, 2014

Suggestion: Support ES6 'let' and 'const' keyword #19

Closed

Add ScriptTarget.ES6 #595

Closed

Noel Abrahams (NoelAbrahams) mentioned this pull request

Jan 12, 2015

Allow pure ECMAScript block #1644

Closed

Adi Dahiya (adidahiya) mentioned this pull request

Apr 20, 2015

ES6 let and const syntax palantir/tslint#371

Closed

Nikolaos S. Papaspyrou (nickie) commented Feb 1, 2016

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TypeScript's specification seems to allow a missing initializer for destructuring lexical bindings, whereas the EC262 specification does not. Is this intentional? In that case, I believe it should only be allowed in the case that a type annotation is present, e.g., in let [x, y]: number[]; The current playground implementation seems to disallow this.

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Nathan Shively-Sanders (sandersn) commented Feb 1, 2016

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Nikolaos S. Papaspyrou (@nickie), can you report a new bug for this, with the label Spec?

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Nikolaos S. Papaspyrou (nickie) commented Feb 1, 2016

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Nathan Shively-Sanders (@sandersn), done (6785) but I don't know how to add the label. Please add it, or advise me how to. Thanks.

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Ryan Cavanaugh (RyanCavanaugh) commented Feb 1, 2016

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Nathan Shively-Sanders (@sandersn) FYI only Owners (i.e. us) can add Labels

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Paul Jolly (myitcv) mentioned this pull request

Apr 7, 2016

let variable shadowing let variable of same name cannot reference outer variable in initialiser #7939

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Microsoft (microsoft) locked and limited conversation to collaborators

Jun 18, 2018

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