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repl: assign underscore fix #3737

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repl: assign underscore fix #3737

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bcfa885
repl: pre-define `_` in REPL's context
thefourtheye Nov 10, 2015
c2a3731
test: make sure `_` cannot be defined in REPL
thefourtheye Nov 10, 2015
48d79a4
doc: warn about reassigning _ in REPL
thefourtheye Nov 10, 2015
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4 changes: 4 additions & 0 deletions doc/api/repl.markdown
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Expand Up @@ -86,6 +86,10 @@ The special variable `_` (underscore) contains the result of the last expression
4
```

*NOTE*: Explicitly assigning a value to `_` in the REPL can produce unexpected
results. Also, attempting to create `_` as a `const` variable in the REPL will
fail.

The REPL provides access to any variables in the global scope. You can expose
a variable to the REPL explicitly by assigning it to the `context` object
associated with each `REPLServer`. For example:
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6 changes: 6 additions & 0 deletions lib/repl.js
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Expand Up @@ -562,6 +562,12 @@ REPLServer.prototype.createContext = function() {
});
});

// Refer: https://github.com/nodejs/node/pull/3729#issuecomment-155460861
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Instead of referring to an issue, how about just explaining why we're doing this.

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Adding here as well.

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+1 to @cjihrig's comment. The reference to the issue is unnecessary, I think.

// The REPL stores the result of the last evaluated expression in context's _.
// But, in the REPL, if _ is defined as a const, then it will break the REPL.
// So, we define _ first, so that later redefiniitions will fail.
context._ = undefined;

return context;
};

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8 changes: 8 additions & 0 deletions test/parallel/test-repl.js
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Expand Up @@ -94,6 +94,14 @@ function error_test() {
});

send_expect([
// Ref: https://github.com/nodejs/node/pull/3729#issuecomment-155460861
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Same comment here.

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Sure, adding it.

// REPL stores the result of the last evaluated expression in _.
// This test makes sure that _ can not be redefined in REPL.
{ client: client_unix, send: 'const _ = 1',
expect: /^TypeError: Identifier '_' has already been declared/ },
// `_` should still be assignable
{ client: client_unix, send: '_ = 1\n_',
expect: `1\n${prompt_unix}1\n${prompt_unix}` },
// Uncaught error throws and prints out
{ client: client_unix, send: 'throw new Error(\'test error\');',
expect: /^Error: test error/ },
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