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util: display constructor when inspecting objects
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This commit modifies util.inspect(obj) to additionally show the name of
the function that constructed the object. This often reveals useful
information about the object's prototype. In other words, instead of

> new Cls
{}

we have

> new Cls
Cls {}

This also works with exotic objects:

> class ArrayCls extends Array {}
> new ArrayCls(1, 2, 3)
ArrayCls [ 1, 2, 3 ]

The names of "trivial" constructors like Object and Array are not shown,
unless there is a mismatch between the object representation and the
prototype:

> Object.create([])
Array {}

This feature is inspired by browser devtools.

PR-URL: #1935
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
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monsanto authored and rvagg committed Aug 17, 2015
1 parent 94b765f commit 7606bdb
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Showing 3 changed files with 71 additions and 5 deletions.
31 changes: 28 additions & 3 deletions lib/util.js
Original file line number Diff line number Diff line change
Expand Up @@ -167,6 +167,22 @@ function arrayToHash(array) {
}


function getConstructorOf(obj) {
while (obj) {
var descriptor = Object.getOwnPropertyDescriptor(obj, 'constructor');
if (descriptor !== undefined &&
typeof descriptor.value === 'function' &&
descriptor.value.name !== '') {
return descriptor.value;
}

obj = Object.getPrototypeOf(obj);
}

return null;
}


function inspectPromise(p) {
Debug = Debug || require('vm').runInDebugContext('Debug');
var mirror = Debug.MakeMirror(p, true);
Expand Down Expand Up @@ -260,14 +276,17 @@ function formatValue(ctx, value, recurseTimes) {
}
}

var constructor = getConstructorOf(value);
var base = '', empty = false, braces, formatter;

if (Array.isArray(value)) {
if (constructor === Array)
constructor = null;
braces = ['[', ']'];
empty = value.length === 0;
formatter = formatArray;
} else if (value instanceof Set) {
braces = ['Set {', '}'];
braces = ['{', '}'];
// With `showHidden`, `length` will display as a hidden property for
// arrays. For consistency's sake, do the same for `size`, even though this
// property isn't selected by Object.getOwnPropertyNames().
Expand All @@ -276,7 +295,7 @@ function formatValue(ctx, value, recurseTimes) {
empty = value.size === 0;
formatter = formatSet;
} else if (value instanceof Map) {
braces = ['Map {', '}'];
braces = ['{', '}'];
// Ditto.
if (ctx.showHidden)
keys.unshift('size');
Expand All @@ -286,9 +305,11 @@ function formatValue(ctx, value, recurseTimes) {
// Only create a mirror if the object superficially looks like a Promise.
var promiseInternals = value instanceof Promise && inspectPromise(value);
if (promiseInternals) {
braces = ['Promise {', '}'];
braces = ['{', '}'];
formatter = formatPromise;
} else {
if (constructor === Object)
constructor = null;
braces = ['{', '}'];
empty = true; // No other data than keys.
formatter = formatObject;
Expand Down Expand Up @@ -336,6 +357,10 @@ function formatValue(ctx, value, recurseTimes) {
base = ' ' + '[Boolean: ' + formatted + ']';
}

// Add constructor name if available
if (base === '' && constructor)
braces[0] = constructor.name + ' ' + braces[0];

if (empty === true) {
return braces[0] + base + braces[1];
}
Expand Down
2 changes: 1 addition & 1 deletion test/parallel/test-sys.js
Original file line number Diff line number Diff line change
Expand Up @@ -17,7 +17,7 @@ assert.equal(new Date('2010-02-14T12:48:40+01:00').toString(),
assert.equal("'\\n\\u0001'", common.inspect('\n\u0001'));

assert.equal('[]', common.inspect([]));
assert.equal('{}', common.inspect(Object.create([])));
assert.equal('Array {}', common.inspect(Object.create([])));
assert.equal('[ 1, 2 ]', common.inspect([1, 2]));
assert.equal('[ 1, [ 2, 3 ] ]', common.inspect([1, [2, 3]]));

Expand Down
43 changes: 42 additions & 1 deletion test/parallel/test-util-inspect.js
Original file line number Diff line number Diff line change
Expand Up @@ -61,7 +61,7 @@ assert.ok(ex.indexOf('[message]') != -1);

// GH-1941
// should not throw:
assert.equal(util.inspect(Object.create(Date.prototype)), '{}');
assert.equal(util.inspect(Object.create(Date.prototype)), 'Date {}');

// GH-1944
assert.doesNotThrow(function() {
Expand Down Expand Up @@ -306,3 +306,44 @@ checkAlignment(function() {
}());
checkAlignment(new Set(big_array));
checkAlignment(new Map(big_array.map(function(y) { return [y, null]; })));


// Test display of constructors

class ObjectSubclass {}
class ArraySubclass extends Array {}
class SetSubclass extends Set {}
class MapSubclass extends Map {}
class PromiseSubclass extends Promise {}

var x = new ObjectSubclass();
x.foo = 42;
assert.equal(util.inspect(x),
'ObjectSubclass { foo: 42 }');
assert.equal(util.inspect(new ArraySubclass(1, 2, 3)),
'ArraySubclass [ 1, 2, 3 ]');
assert.equal(util.inspect(new SetSubclass([1, 2, 3])),
'SetSubclass { 1, 2, 3 }');
assert.equal(util.inspect(new MapSubclass([['foo', 42]])),
'MapSubclass { \'foo\' => 42 }');
assert.equal(util.inspect(new PromiseSubclass(function() {})),
'PromiseSubclass { <pending> }');

// Corner cases.
var x = { constructor: 42 };
assert.equal(util.inspect(x), '{ constructor: 42 }');

var x = {};
Object.defineProperty(x, 'constructor', {
get: function() {
throw new Error('should not access constructor');
},
enumerable: true
});
assert.equal(util.inspect(x), '{ constructor: [Getter] }');

var x = new (function() {});
assert.equal(util.inspect(x), '{}');

var x = Object.create(null);
assert.equal(util.inspect(x), '{}');

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