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REPL - after failed assignment identifier cannot be reused #3704

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SamuelMarks opened this issue Nov 7, 2015 · 3 comments
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REPL - after failed assignment identifier cannot be reused #3704

SamuelMarks opened this issue Nov 7, 2015 · 3 comments
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repl Issues and PRs related to the REPL subsystem.

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@SamuelMarks
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A:\floppy>node -r lodash
> const _ = require('lodash');
const _ = require('lodash');
TypeError: Cannot assign to read only property '_' of #<Object>
    at finish (repl.js:438:24)
    at REPLServer.defaultEval (repl.js:185:5)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.<anonymous> (repl.js:393:12)
    at emitOne (events.js:82:20)
    at REPLServer.emit (events.js:169:7)
    at REPLServer.Interface._onLine (readline.js:210:10)
    at REPLServer.Interface._line (readline.js:549:8)
    at REPLServer.Interface._ttyWrite (readline.js:826:14)
>
> ;
TypeError: Cannot assign to read only property '_' of #<Object>
    at finish (repl.js:438:24)
    at REPLServer.defaultEval (repl.js:185:5)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.<anonymous> (repl.js:393:12)
    at emitOne (events.js:82:20)
    at REPLServer.emit (events.js:169:7)
    at REPLServer.Interface._onLine (readline.js:210:10)
    at REPLServer.Interface._line (readline.js:549:8)
    at REPLServer.Interface._ttyWrite (readline.js:826:14)
> let _ = require('lodash');
TypeError: Identifier '_' has already been declared
    at repl:1:1
    at REPLServer.defaultEval (repl.js:164:27)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.<anonymous> (repl.js:393:12)
    at emitOne (events.js:82:20)
    at REPLServer.emit (events.js:169:7)
    at REPLServer.Interface._onLine (readline.js:210:10)
    at REPLServer.Interface._line (readline.js:549:8)
    at REPLServer.Interface._ttyWrite (readline.js:826:14)
> process.version
TypeError: Cannot assign to read only property '_' of #<Object>
    at finish (repl.js:438:24)
    at REPLServer.defaultEval (repl.js:185:5)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.<anonymous> (repl.js:393:12)
    at emitOne (events.js:82:20)
    at REPLServer.emit (events.js:169:7)
    at REPLServer.Interface._onLine (readline.js:210:10)
    at REPLServer.Interface._line (readline.js:549:8)
    at REPLServer.Interface._ttyWrite (readline.js:826:14)

I am running Node 4.2.1 x64 on Windows 8.1 x64.
@thefourtheye thefourtheye added the repl Issues and PRs related to the REPL subsystem. label Nov 7, 2015
@thefourtheye
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@SamuelMarks _ is used internally to track the result of the last evaluated expression. Please check this section of the REPL doc. Since you are making it a const, internally, when REPL tries to store the result on _, it fails because it is read-only now. So, I don't think this is a bug per-se and closing the issue. Please feel free to comment if you have strong reasons to believe that this should be fixed and we can reopen this and find a solution.

@SamuelMarks
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Oh right, forgot about that. When I was coding Node years ago I now remember setting l = require('lodash').

@thefourtheye thefourtheye mentioned this issue Nov 10, 2015
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@thefourtheye
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@SamuelMarks No problem :-) I created a PR (#3729) with doc changes regarding the same. Please feel free to review it.

@thefourtheye thefourtheye mentioned this issue Nov 10, 2015
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thefourtheye added a commit to thefourtheye/io.js that referenced this issue Nov 10, 2015
@thefourtheye
doc: warn about reassigning _ in REPL
6417f8b 
When users assign a value to `_` in REPL, it is prone to unexpected
results or messing up with REPL's internals. For example,
nodejs#3704. This patch issues a warning
about the same.
thefourtheye added a commit to thefourtheye/io.js that referenced this issue Feb 25, 2016
@thefourtheye
repl: pre-define _ in REPL's context
bcfa885 
As `_` is not defined in REPL's context, when it is defined as `const`,
it breaks REPL, as it tries to store the result of the last evaluated
expression in `_`. This patch makes sure that `_` is pre-defined in
REPL's context, so that if users define it again, they will get error.

Refer: nodejs#3729
Refer: nodejs#3704
thefourtheye added a commit to thefourtheye/io.js that referenced this issue Feb 25, 2016
@thefourtheye
test: make sure _ cannot be defined in REPL
c2a3731 
If the `_` is redefined as `const` in REPL, it will break the REPL, as
REPL will store the result of the last evaluated expression in `_`.
This patch has a test to make sure that the REPL doesn't allow
redefining `_` as `const`, also still assiging values to `_` is
permitted.

Refer: nodejs#3729
Refer: nodejs#3704
thefourtheye added a commit to thefourtheye/io.js that referenced this issue Feb 25, 2016
@thefourtheye
doc: warn about reassigning _ in REPL
48d79a4 
When users assign a value to `_` in REPL, it is prone to unexpected
results or messing up with REPL's internals. For example,
nodejs#3704. This patch issues a warning
about the same.
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@thefourtheye @SamuelMarks