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util: allow symbol-based custom inspection methods #8174

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util: allow symbol-based custom inspection methods #8174

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util: allow symbol-based custom inspection methods
addaleax Aug 18, 2016
2dc6163
util: allow returning `this` from custom inspect
addaleax Aug 18, 2016
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util: allow returning this from custom inspect 
If a custom inspection function returned `this`, use that value
for further formatting instead of going into infinite recursion.

This is particularly useful when combined with `util.inspect.custom`
because returning `this` from such a method makes it easy to
have an `inspect()` function that is ignored by `util.inspect` without
actually having to provide an alternative for custom inspection.
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@addaleax
addaleax committed Aug 24, 2016
commit 2dc6163d7cd630494c9f16215cf1abba35c56c87
11 changes: 8 additions & 3 deletions lib/util.js
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Original file line number Diff line number Diff line change
Expand Up @@ -362,10 +362,15 @@ function formatValue(ctx, value, recurseTimes) {
// Also filter out any prototype objects using the circular check.
!(value.constructor && value.constructor.prototype === value)) {
let ret = maybeCustomInspect.call(value, recurseTimes, ctx);
if (typeof ret !== 'string') {
ret = formatValue(ctx, ret, recurseTimes);

// If the custom inspection method returned `this`, don't go into
// infinite recursion.
if (ret !== value) {
if (typeof ret !== 'string') {
ret = formatValue(ctx, ret, recurseTimes);
}
return ret;
}
return ret;
}
}

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10 changes: 10 additions & 0 deletions test/parallel/test-util-inspect.js
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Expand Up @@ -567,6 +567,16 @@ assert.doesNotThrow(function() {
);
}

{
// Returning `this` from a custom inspection function works.
assert.strictEqual(util.inspect({ a: 123, inspect() { return this; } }),
'{ a: 123, inspect: [Function: inspect] }');

const subject = { a: 123, [util.inspect.custom]() { return this; } };
assert.strictEqual(util.inspect(subject),
'{ a: 123 }');
}

// util.inspect with "colors" option should produce as many lines as without it
function test_lines(input) {
var count_lines = function(str) {
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