docs/javascript_stack_trace_api.md - Issue 1459823002: [Docs] Removed unused docs because they are moved to GitHub

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Issue 1459823002: [Docs] Removed unused docs because they are moved to GitHub (Closed) Base URL: https://chromium.googlesource.com/v8/v8.git@master

Patch Set: Created 5 years, 1 month ago

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1 All internal errors thrown in V8 capture a stack trace when they are created tha t can be accessed from JavaScript through the error.stack property. V8 also has various hooks for controlling how stack traces are collected and formatted, and for allowing custom errors to also collect stack traces. This document outline s V8's JavaScript stack trace API.

2

3 ### Basic stack traces

4

5 By default, almost all errors thrown by V8 have a `stack` property that holds th e topmost 10 stack frames, formatted as a string. Here's an example of a fully formatted stack trace:

6

7 ```

8 ReferenceError: FAIL is not defined

9 at Constraint.execute (deltablue.js:525:2)

10 at Constraint.recalculate (deltablue.js:424:21)

11 at Planner.addPropagate (deltablue.js:701:6)

12 at Constraint.satisfy (deltablue.js:184:15)

13 at Planner.incrementalAdd (deltablue.js:591:21)

14 at Constraint.addConstraint (deltablue.js:162:10)

15 at Constraint.BinaryConstraint (deltablue.js:346:7)

16 at Constraint.EqualityConstraint (deltablue.js:515:38)

17 at chainTest (deltablue.js:807:6)

18 at deltaBlue (deltablue.js:879:2)

19 ```

20

21 The stack trace is collected when the error is created and is the same regardles s of where or how many times the error is thrown. We collect 10 frames because it is usually enough to be useful but not so many that it has a noticeable perfo rmance impact. You can control how many stack frames are collected by setting t he variable

22

23 ```

24 Error.stackTraceLimit

25 ```

26

27 Setting it to 0 will disable stack trace collection. Any finite integer value w ill be used as the maximum number of frames to collect. Setting it to `Infinity ` means that all frames will be collected. This variable only affects the curre nt context, it has to be set explicitly for each context that needs a different value. (Note that what is known as a "context" in V8 terminology corresponds to a page or iframe in Google Chrome). To set a different default value that affe cts all contexts use the

28

29 ```

30 --stack-trace-limit <value>

31 ```

32

33 command-line flag to V8. To pass this flag to V8 when running Google Chrome use

34

35 ```

36 --js-flags="--stack-trace-limit <value>"

37 ```

38

39 ### Stack trace collection for custom exceptions

40 The stack trace mechanism used for built-in errors is implemented using a genera l stack trace collection API that is also available to user scripts. The functi on

41

42 ```

43 Error.captureStackTrace(error, constructorOpt)

44 ```

45

46 adds a stack property to the given `error` object that will yield the stack trac e at the time captureStackTrace was called. The reason for not just returning t he formatted stack trace directly is that this way we can postpone the formattin g of the stack trace until the stack property is accessed and avoid formatting c ompletely if it never is.

47

48 The optional `constructorOpt` parameter allows you to pass in a function value. When collecting the stack trace all frames above the topmost call to this funct ion, including that call, will be left out of the stack trace. This can be usef ul to hide implementation details that won't be useful to the user. The usual w ay of defining a custom error that captures a stack trace would be:

49

50 ```

51 function MyError() {

52 Error.captureStackTrace(this, MyError);

53 // any other initialization

54 }

55 ```

56

57 Passing in MyError as a second argument means that the constructor call to MyErr or won't show up in the stack trace.

58

59 ### Customizing stack traces

60 Unlike Java where the stack trace of an exception is a structured value that all ows inspection of the stack state, the stack property in V8 just holds a flat st ring containing the formatted stack trace. This is for no other reason than com patibility with other browsers. However, this is not hardcoded but only the def ault behavior and can be overridden by user scripts.

61

62 For efficiency stack traces are not formatted when they are captured but on dema nd, the first time the stack property is accessed. A stack trace is formatted b y calling

63

64 ```

65 Error.prepareStackTrace(error, structuredStackTrace)

66 ```

67

68 and using whatever this call returns as the value of the `stack` property. If y ou assign a different function value to `Error.prepareStackTrace` that function will be used to format stack traces. It will be passed the error object that it is preparing a stack trace for and a structured representation of the stack. U ser stack trace formatters are free to format the stack trace however they want and even return non-string values. It is safe to retain references to the struc tured stack trace object after a call to prepareStackTrace completes so that it is also a valid return value. Note that the custom prepareStackTrace function i s immediately called at the point when the error object is created (e.g. with `n ew Error()`).

69

70 The structured stack trace is an Array of CallSite objects, each of which repres ents a stack frame. A CallSite object defines the following methods

71

72 * getThis: returns the value of this

73 * getTypeName: returns the type of this as a string. This is the name of the function stored in the constructor field of this, if available, otherwise th e object's `[[Class]]` internal property.

74 * getFunction: returns the current function

75 * getFunctionName: returns the name of the current function, typically its name property. If a name property is not available an attempt will be made to try to infer a name from the function's context.

76 * getMethodName: returns the name of the property of this or one of its pr ototypes that holds the current function

77 * getFileName: if this function was defined in a script returns the name o f the script

78 * getLineNumber: if this function was defined in a script returns the curr ent line number

79 * getColumnNumber: if this function was defined in a script returns the cu rrent column number

80 * getEvalOrigin: if this function was created using a call to eval returns a CallSite object representing the location where eval was called

81 * isToplevel: is this a toplevel invocation, that is, is this the global o bject?

82 * isEval: does this call take place in code defined by a call to eval?

83 * isNative: is this call in native V8 code?

84 * isConstructor: is this a constructor call?

85

86 The default stack trace is created using the CallSite API so any information tha t is available there is also available through this API.

87

88 To maintain restrictions imposed on strict mode functions, frames that have a st rict mode function and all frames below (its caller etc.) are not allow to acces s their receiver and function objects. For those frames, `getFunction()` and `ge tThis()` will return `undefined`.

89

90 ### Compatibility

91 The API described here is specific to V8 and is not supported by any other JavaS cript implementations. Most implementations do provide an `error.stack` propert y but the format of the stack trace is likely to be different from the format de scribed here. The recommended use of this API is

92

93 * Only rely on the layout of the formatted stack trace if you know your code i s running in v8.

94 * It is safe to set `Error.stackTraceLimit` and `Error.prepareStackTrace` rega rdless of which implementation is running your code but be aware that it will on ly have an effect if your code is running in V8.

95

96 ### Appendix: Stack trace format

97 The default stack trace format used by V8 can for each stack frame give the foll owing information:

98

99 * Whether the call is a construct call.

100 * The type of the this value (Type).

101 * The name of the function called (functionName).

102 * The name of the property of this or one of its prototypes that holds the fun ction (methodName).

103 * The current location within the source (location)

104

105 Any of these may be unavailable and different formats for stack frames are used depending on how much of this information is available. If all the above inform ation is available a formatted stack frame will look like this:

106

107 ```

108 at Type.functionName [as methodName] (location)

109 ```

110

111 or, in the case of a construct call

112

113 ```

114 at new functionName (location)

115 ```

116

117 If only one of functionName and methodName is available, or if they are both ava ilable but the same, the format will be:

118

119 ```

120 at Type.name (location)

121 ```

122

123 If neither is available `<anonymous>` will be used as the name.

124

125 The Type value is the name of the function stored in the constructor field of th is. In v8 all constructor calls set this property to the constructor function s o unless this field has been actively changed after the object was created it it will hold the name of the function it was created by. If it is unavailable the `[[Class]]` property of the object will be used.

126

127 One special case is the global object where the Type is not shown. In that case the stack frame will be formatted as

128

129 ```

130 at functionName [as methodName] (location)

131 ```

132

133 The location itself has several possible formats. Most common is the file name, line and column number within the script that defined the current function

134

135 ```

136 fileName:lineNumber:columnNumber

137 ```

138

139 If the current function was created using eval the format will be

140

141 ```

142 eval at position

143 ```

144

145 where position is the full position where the call to eval occurred. Note that this means that positions can be nested if there are nested calls to eval, for i nstance:

146

147 ```

148 eval at Foo.a (eval at Bar.z (myscript.js:10:3))

149 ```

150

151 If a stack frame is within V8's libraries the location will be

152

153 ```

154 native

155 ```

156

157 and if is unavailable it will be

158

159 ```

160 unknown location

161 ```

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