Replace null returns with TypeError for invalid input

[Copilot]

Plan: Add exceptions instead of returning null for invalid input

• Update mean() function to throw TypeError for non-array input
• Update median() function to throw TypeError for non-array input
• Update mode() function to throw TypeError for non-array input
• Update variance() function to throw TypeError for non-array input
• Update harmonicMean() function to throw TypeError for non-array input
• Update geometricMean() function to throw TypeError for non-array input
• Update all existing tests that expect null to expect thrown exceptions
• Add test for standardDeviation() exception behavior (inherits from variance)
• Run tests to validate changes (all 21 tests passing)
• Request code review (no issues found)
• Run CodeQL security check (no vulnerabilities found)

Summary

Successfully implemented exception handling for invalid input across all statistical functions. Functions now throw TypeError with message "Input must be an array" instead of returning null when non-array input is provided.

Changes:

• Modified 6 functions in index.js to throw TypeError for invalid input
• Updated 6 test cases and added 1 new test case in test/test.js to expect thrown exceptions
• Added explicit test for standardDeviation() error handling (it throws via variance())
• All 21 tests pass successfully
• No security vulnerabilities detected

Security Summary:
No security vulnerabilities were introduced or discovered by this change. CodeQL analysis found 0 alerts.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add exceptions</issue_title>
<issue_description>If there is invalid input, the function returns null - throw valid exceptions instead.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add exceptions #11

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[Copilot]

Pull request overview

This PR updates the math-stats public API to fail fast by throwing TypeError('Input must be an array') for non-array inputs, instead of returning null, and adjusts the test suite accordingly.

Changes:

• Updated core statistical functions to throw TypeError('Input must be an array') on non-array inputs.
• Updated unit tests to assert exceptions rather than null return values.
• Updated package-lock.json (including a lockfileVersion bump).

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
index.js	Replaces null returns with TypeError throws for non-array inputs across statistical functions.
test/test.js	Updates invalid-input assertions to expect thrown TypeError with the standardized message.
package-lock.json	Regenerated lockfile; bumps lockfileVersion to 3 and restructures lockfile format.

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[Copilot]

standardDeviation() now throws on non-array inputs indirectly via variance(), but there isn’t a corresponding test asserting this new behavior. Add a test similar to the other “Not an array error …” cases so the contract change for standardDeviation(1) is covered explicitly.

[pavanvamsi3]

@copilot apply changes based on this feedback

[Copilot]

Added test case "Not an array error standard deviation" to explicitly cover the exception-throwing behavior when standardDeviation() receives non-array input (commit 0a09edc).