1. The no undef error occurs because there cannot be undefined variables in the syntax detection of eslint. Add them to the rules {} of. Eslintrc.js in the root directory   ‘ no-undef’: 0

Then restart the editor.

2. Rule configuration of eslint

In. Eslintrc.js, the rules of rules are as follows:

rules: {
    "RuleName": [RuleValue, RuleConfig]
}
Rule value:
"off" or 0 // turn off detection rules
"warn" or 1 // turn on and treat the open detection rule as a warning (does not affect the exit code)
"error" or 2 // turn on and treat the detection rule as an error (1 when the exit code is triggered)

The eslint rules are as follows:

"no-alert": 0,//prohibit the use of alert confirm prompt
"no-array-constructor": 2,//prohibit the use of array constructors
"no-bitwise": 0,//prohibit the use of bitwise operators
"no-caller": 1,//prohibit the use of arguments.caller or arguments.callee
"no-catch-shadow": 2,//prohibit the use of catch clause arguments with the same name as external scope variables
"no-class-assign": 2,//forbid to assign a value to a class
"no-cond-assign": 2,//prohibit the use of assignment statements in conditional expressions
"no-console": 2,//Disable the use of consoles
"no-const-assign": 2,//forbid to modify variables declared by const
"no-constant-condition": 2,//prohibit the use of constant expressions in conditions if(true) if(1)
"no-continue": 0,//prohibit the use of continue
"no-control-regex": 2,//prohibit the use of control characters in regular expressions
"no-debugger": 2,//Debugger is forbidden
"no-delete-var": 2,//do not use the delete operator for variables declared by var
"no-div-regex": 1,//can't use regular expressions that look like division /=foo/
"no-dupe-keys": 2,// do not allow duplicate keys when creating object literals {a:1,a:1}
"no-dupe-args": 2,// function arguments cannot be duplicated
"no-duplicate-case": 2,//Switch case tags cannot be duplicated
"no-else-return": 2,//If there is a return in the if statement, it can't be followed by an else statement
"no-empty": 2,//Block statement can not be empty
"no-empty-character-class": 2,//the content of [] in a regular expression cannot be empty
"no-empty-label": 2,//prohibit the use of empty labels
"no-eq-null": 2,//Disable the use of == or ! = operator
"no-eval": 1,//forbid the use of eval
"no-ex-assign": 2,//forbid to assign values to exception parameters in catch statements
"no-extend-native": 2,//prohibit extending native objects
"no-extra-bind": 2,//Disable unnecessary function binding
"no-extra-boolean-cast": 2,//Disable unnecessary bool conversions
"no-extra-parens": 2,//prohibit unnecessary parentheses
"no-extra-semi": 2,//prohibit superfluous colons
"no-fallthrough": 1,//prohibit switch pass-through
"no-floating-decimal": 2,//Disable omitting 0 .5 in floating point numbers 3.
"no-func-assign": 2,//Disable duplicate function declarations
"no-implicit-coercion": 1,//prohibit implicit conversions
"no-implied-eval": 2,//prohibit the use of implicit eval
"no-inline-comments": 0,//prohibit inline comments
"no-inline-declarations": [2, "functions"],//prohibit the use of declarations (variables or functions) in block statements
"no-invalid-regexp": 2,//Disable invalid regular expressions
"no-invalid-this": 2,//prohibit invalid this, only for constructors, classes, object literals
"no-irregular-whitespace": 2,//no irregular spaces
"no-iterator": 2,//prohibit the use of __iterator__ attribute
"no-label-var": 2,// the name of a label cannot be the same as the name of a variable declared by var
"no-labels": 2,//prohibit label declarations
"no-lone-blocks": 2,//Disable unnecessary nested blocks
"no-lonely-if": 2,//prohibit only if statements inside an else statement
"no-loop-func": 1,//prohibit the use of functions in loops (if they do not refer to external variables and do not form closures)
"no-mixed-requires": [0, false],//No mixed declaration types when declaring
"no-mixed-spaces-and-tabs": [2, false],//prohibit mixing tabs and spaces
"linebreak-style": [0, "windows"],//line break style
"no-multi-spaces": 1,//no extra spaces
"no-multi-str": 2,//Strings can't use \ newlines
"no-multiple-empty-lines": [1, {"max": 2}],//no more than 2 empty lines
"no-native-reassign": 2,//no native objects can be rewritten
"no-negated-in-lhs": 2,// the left side of the in operator cannot have !
"no-nested-ternary": 0,//Nested trinomials are forbidden
"no-new": 1,//forbid to construct an instance using new without assigning a value
"no-new-func": 1,//Disable the use of new Function
"no-new-object": 2,//Disable the use of new Object()
"no-new-require": 2,//prohibit the use of new require
"no-new-wrappers": 2,//prohibit the use of new to create wrapper instances, new String new Boolean new Number
"no-obj-calls": 2,//can't call built-in global objects, such as Math() JSON()
"no-octal": 2,//forbid the use of octal numbers
"no-octal-escape": 2,//forbid the use of octal escape sequences
"no-param-reassign": 2,//Disable reassigning parameters
"no-path-concat": 0,//do not use __dirname or __filename for path concatenation in node
"no-plusplus": 0,//Prohibit the use of ++, --
"no-process-env": 0,//prohibit the use of process.env
"no-process-exit": 0,//prohibit the use of process.exit()
"no-proto": 2,//prohibit the use of __proto__ attribute
"no-redeclare": 2,//prohibit repeated declaration of variables
"no-regex-spaces": 2,//prohibit the use of multiple spaces in regular expression literals /foo bar/
"no-restricted-modules": 0,//If the specified module is disabled, an error will be reported if it is used
"no-return-assign": 1,//return statements cannot have assignment expressions in them
"no-script-url": 0,//prohibit the use of javascript:void(0)
"no-self-compare": 2,//can't compare itself
"no-sequences": 0,//forbid the use of comma operators
"no-shadow": 2,//Variables in external scopes cannot have the same name as variables or arguments in the scope they are contained in
"no-shadow-restricted-names": 2,//restricted identifiers specified in strict mode cannot be used as variable names at declaration time
"no-spaced-func": 2,//Function call with no spaces between function name and ()
"no-sparse-arrays": 2,//Sparse arrays are forbidden, [1,,2]
"no-sync": 0,//nodejs disables sync methods
"no-ternary": 0,//prohibit the trinomial operator
"no-tiling-spaces": 1,//don't have spaces after the end of a line
"no-this-before-super": 0,//no use of this or super before calling super()
"no-throw-literal": 2,//Disable throwing literal errors throw "error";
"no-undef": 1,//no undefined variables
"no-undef-init": 2,//variables can't be assigned to undefined when initialized
"no-undefined": 2,//can't use undefined
"no-unexpected-multiline": 2,//Avoid multi-line expressions
"no-underscore-dangle": 1,//Identifiers cannot start or end with _
"no-unneeded-ternary": 2,//prohibit unnecessary nesting var isYes = answer === 1 ?true : false;
"no-unreachable": 2,//no unexecutable code
"no-unused-expressions": 2,//prohibit unused expressions
"no-unused-vars": [2, {"vars": "all", "args": "after-used"}],//no variables or arguments that are declared and not used
"no-use-before-define": 2,//no-use-before-define
"no-useless-call": 2,//prohibit unnecessary calls and applies
"no-void": 2,//Disable void operator
"no-var": 0,//disable var and replace it with let and const
"no-warning-comments": [1, { "terms": ["todo", "fixme", "xxx"], "location": "start" }],//can't have warning comments
"no-with": 2,//disable with

"array-bracket-spacing": [2, "never"],//Whether to allow non-empty arrays with extra spaces inside
"arrow-parens": 0,//arrow function enclosed in parentheses
"arrow-spacing": 0,//=>s pre/post brackets
"accessor-pairs": 0,//use getter/setter in objects
"block-scoped-var": 0,//use var in block statements
"brace-style": [1, "1tbs"],//brace style
"callback-return": 1,//avoid multiple calls to callbacks and stuff
"camelcase": 2,//force camelcase naming
"comma-dangle": [2, "never"],//no comma at the end of object literals
"comma-spacing": 0,//space before and after the comma
"comma-style": [2, "last"],//comma style, at the beginning or at the end of a line when a line break occurs
"complexity": [0, 11],//complexity of the loop
"computed-property-spacing": [0, "never"],//whether to allow computed key names or whatever
"consistent-return": 0,//whether to allow omission after return
"consistent-this": [2, "that"],//this alias
"constructor-super": 0,//non-derived class cannot call super, derived class must call super
"curly": [2, "all"],//must use {} in if(){}
"default-case": 2,//switch statement must have default at the end
"dot-location": 0,//the location of the object accessor, whether it is at the beginning or the end of the line when a line break occurs
"dot-notation": [0, { "allowKeywords": true }],//avoid unnecessary square brackets
"eol-last": 0,//file ends with a single newline character
"eqeqeq": 2,//must use all-equal
"func-names": 0,//Function expressions must have names
"func-style": [0, "declaration"],//function style, specifying that only function declarations/function expressions can be used
"generator-star-spacing": 0,// the space before and after the generator function*
"guard-for-in": 0,//for in loop to be filtered with if statement
"handle-callback-err": 0,//nodejs handles errors
"id-length": 0,//the length of the variable name
"indent": [2, 4],//indent style
"init-declarations": 0,//must assign initial values when declaring
"key-spacing": [0, { "beforeColon": false, "afterColon": true }],//space before and after the colon in the object literal
"lines-around-comment": 0, // before/ after line comment
"max-depth": [0, 4],// depth of nested blocks
"max-len": [0, 80, 4],//maximum length of string
"max-nested-callbacks": [0, 2],//Nesting depth of callbacks
"max-params": [0, 3],//the maximum number of parameters a function can have is 3
"max-statements": [0, 10],//the maximum number of statements within the function
"new-cap": 2,//the first line of the function name must be called in upper case using new, the first line must be called in lower case without new
"new-parens": 2,//new must be called with parentheses
"newline-after-var": 2,//whether a blank line is required after variable declaration
"object-curly-spacing": [0, "never"],//whether unnecessary spaces are allowed inside curly brackets
"object-shorthand": 0,// force object literal abbreviation syntax
"one-var": 1,//continuous declaration
"operator-assignment": [0, "always"],//assignment operator += -= or something
"operator-linebreak": [2, "after"],//whether the operator is at the end or the beginning of a line when a line break occurs
"padded-blocks": 0,//whether to have empty lines at the beginning and end of a block statement
"prefer-const": 0,//prefer-const
"prefer-spread": 0,//prefer spread
"prefer-reflect": 0,//prefer the method of reflect
"quotes": [1, "single"],//quote type `` "" ''
"quote-props":[2, "always"],// whether to force double quotes for property names in object literals
"radix": 2,//parseInt must specify the second parameter
"id-match": 0,//name detection
"require-yield": 0,//generator function must have yield
"semi": [2, "always"],//statement forced to end with a semicolon
"semi-spacing": [0, {"before": false, "after": true}],//spaces before and after the semicolon
"sort-vars": 0,//variable declaration sorting
"space-after-keywords": [0, "always"],//whether to leave a space after the keyword
"space-before-blocks": [0, "always"],//whether to have a space before blocks that do not start with a new line
"space-before-function-parents": [0, "always"],//space before parentheses for function definitions
"space-in-parents": [0, "never"],//space inside parentheses
"space-infix-ops": 0,//whether there should be spaces around the midfix operator
"space-return-throw-case": 2,//would you like a space after the return throw case
"space-unary-ops": [0, { "words": true, "nonwords": false }],//spaces before/after unary operators
"spaced-comment": 0,//comment style should have spaces or something
"strict": 2,//use strict mode
"use-isnan": 2,//forbid to use NaN when comparing, only use isNaN()
"valid-jsdoc": 0,//jsdoc rule
"valid-typeof": 2,//must use legal typeof value
"vars-on-top": 2,//var must be placed at the top of the scope
"wrap-iife": [2, "inside"],//immediately execute the parenthesis style of the function expression
"wrap-regex": 0,// regular expression literals wrapped in parentheses
"yoda": [2, "never"]//prohibit yoda conditions