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Assertions - JavaScript | MDN
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Assertions

(assertion) . , (look-ahead) (look-behind) .

In this article

: ? .
x(?=y) : x x y. , /Jack(?=Sprat)/ "Jack" "Sprat". /Jack(?=Sprat|Frost)/ "Jack" "Sprat" "Frost". , "Sprat" "Frost" .
x(?!y) : x x y. , /\d+(?!\.)/ , . /\d+(?!\.)/.exec('3.141') "141", "3" .
(?<=y)x : x x y. , /(?<=Jack)Sprat/ "Sprat" "Jack". /(?<=Jack|Tom)Sprat/ "Sprat" "Jack" "Tom". , "Jack" "Tom" .
(?<!y)x : x x y. , /(?<!-)\d+/ , . /(?<!-)\d+/.exec('3') "3". /(?<!-)\d+/.exec('-3') , .. .

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    77. SyntaxError: raw bracket is not allowed in regular expression with unicode flag
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    92. :
    93. TypeError: 'x' is not iterable ( 'x' )
    94. TypeError: "x" is (not) "y"
    95. TypeError: "x" is not a constructor
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    98. TypeError: "x" is read-only( : "x" )
    99. TypeError: already executing generator
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    112. TypeError: class constructors must be invoked with 'new'
    113. TypeError: cyclic object value
    114. TypeError: derived class constructor returned invalid value x
    115. TypeError: getting private setter-only property
    116. TypeError: Initializing an object twice is an error with private fields/methods
    117. TypeError: invalid 'instanceof' operand 'x'( : '' '')
    118. TypeError: invalid Array.prototype.sort argument (TypeError: Array.prototype.sort argument)
    119. TypeError: invalid assignment to const "x"( const "x")
    120. TypeError: Iterator/AsyncIterator constructor can't be used directly
    121. TypeError: matchAll/replaceAll must be called with a global RegExp
    122. TypeError: More arguments needed
    123. TypeError: "x" has no properties
    124. TypeError: property "x" is non-configurable and can't be deleted( "x" )
    125. TypeError: Reduce of empty array with no initial value( : )
    126. TypeError: setting getter-only property "x"( "x")
    127. TypeError: WeakSet key/WeakMap value 'x' must be an object or an unregistered symbol
    128. X.prototype.y called on incompatible type
    129. URIError: malformed URI sequence
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    131. Warning: unreachable code after return statement
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