Common Causes and Solutions for JavaScript Error: "Expected identifier, string or number"

Error Mechanism Analysis

In JavaScript development, "Expected identifier, string or number" is a common syntax error message. This error typically occurs when the parser expects to encounter an identifier, string, or number but instead finds other symbols that do not conform to syntactic rules. From an underlying mechanism perspective, JavaScript engines build abstract syntax trees (AST) during code parsing and throw this type of error when encountering unclassifiable tokens.

Primary Cause: Comma Errors in Object Definitions

The most common triggering scenario involves incorrectly placed commas in object literal definitions. Consider the following code example:

var userProfile = {
  userId: 12345,
  userName: "john_doe",
  userEmail: "john@example.com",
};

In this example, the comma following the last property userEmail causes a syntax error in strict mode. While modern JavaScript engines have some tolerance for this, in IE7 and early Firefox versions, such trailing commas explicitly trigger the "Expected identifier, string or number" error.

Potential Risks in Dynamic Object Construction

When errors occur at seemingly random line numbers, they are often related to dynamic object construction. For example:

function createConfig(options) {
  var config = {
    baseUrl: options.url,
    timeout: options.timeout,
  };
  
  // Dynamically add properties
  if (options.retry) {
    config.retryCount = options.retry.count,
    config.retryInterval = options.retry.interval,
  }
  
  return config;
}

When dynamically adding properties in conditional branches, it's easy to make mistakes with comma usage, especially in complex business logic.

Reserved Keyword Usage Issues

Another significant cause involves using JavaScript reserved keywords as object property names. Particularly in Internet Explorer, the use of keywords like class is strictly restricted:

// Incorrect usage - triggers error in IE
var elementStyle = {
  class: "highlighted",
  id: "main-element"
};

// Correct usage - wrap keywords in quotes
var elementStyle = {
  "class": "highlighted",
  "id": "main-element"
};

Debugging and Prevention Strategies

For such issues, developers can adopt the following systematic debugging approaches:

1. Code Review: Focus on comma usage in object literals and array definitions
2. Static Analysis Tools: Use modern tools like ESLint instead of JSLint, configuring the comma-dangle rule
3. Browser Compatibility Testing: Conduct thorough testing in target browser environments
4. Coding Standards: Establish unified code style guidelines for teams, avoiding trailing commas

Modern JavaScript Best Practices

With the evolution of ECMAScript standards, modern JavaScript offers better solutions:

// Using object destructuring and default values
const createUser = ({id, name, email} = {}) => ({
  id: id || generateId(),
  name: name || "Anonymous",
  email: email || ""
});

// Using Map for dynamic key names
const styleMap = new Map();
styleMap.set("class", "container");
styleMap.set("data-id", "unique123");

By adopting these modern features, common syntax errors in traditional object operations can be significantly reduced.