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Freerange is a static analyzer that tracks possible numeric values through supported TypeScript code and can flag hazards such as division by zero, possible NaN or Infinity, and out-of-bounds array reads. It works at build time through the official TypeScript API, without requiring a new language or annotations. Its findings are useful within the code it understands—not a blanket guarantee that an entire application is safe.
What is Freerange?
Freerange analyzes numeric ranges in TypeScript: instead of executing a program with particular inputs, it reasons about the values variables and expressions could have along supported paths. That can help developers spot numeric hazards before runtime. The project’s documentation describes potential findings including NaN, Infinity, division by zero, and array reads that may be out of bounds. Freerange’s repository and documentation explain the tool and its supported code patterns.
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The idea is especially relevant when TypeScript code is produced or changed quickly, including with coding agents. A type checker can verify many type relationships, but numeric constraints such as “this divisor is never zero” are a different question. A range analyzer attempts to infer such constraints from the code it can analyze.
How can it find a division-by-zero path?
The project documentation illustrates a grid-width calculation in which a 200-pixel container can result in zero columns after division and flooring. If later code divides by that column count, the divisor may be zero. The example shows how an innocuous-looking calculation can create a runtime hazard; it is an illustration in the project documentation, not a performance test or independent verification.
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In practical terms, a finding can point to a place where an inferred range permits an unsafe value. That makes the report a prompt to inspect the calculation, its inputs, and the assumptions the surrounding code relies on—not a substitute for understanding the program.
What do fr and fr --audit do?
frreports findings for a project.fr --auditexamines function contracts and offers refactoring suggestions. The audit can also help identify unsupported code, which matters when interpreting what the analyzer has and has not checked.
Freerange uses the official TypeScript API and is designed to fit into a TypeScript project workflow. It does not require adopting a new language or adding annotations to express numeric ranges.
Rank #2
- TypeScript implements a superset of syntax for strictly typed development, facilitating deep static analysis and enhanced development environment integration. The compiler translates source into standard script formats, ensuring parity across any runtime.
- TypeScript is ideal for front-end developers, full-stack engineers, and software architects who build large-scale web applications. It serves those looking to improve code excellence, reduce bugs through static checking, and maintain complex projects more.
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What TypeScript code can Freerange analyze?
Freerange deliberately supports a subset of TypeScript rather than every valid program pattern. Its documented scope includes named, synchronous, top-level functions; supported arithmetic and control flow; and selected object and array patterns. Imported function bodies and callbacks passed to higher-order functions are not analyzed. Object and array writes are unsupported, and other patterns may receive partial or no support.
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This boundary changes what a clean report means. If relevant logic is in unsupported or unanalyzed code, the analyzer cannot establish safety for that logic. A project with no findings is therefore not proof that every behavior is safe; check audit results and the documented limitations to understand coverage.
How precise are its numeric conclusions?
The documentation says Freerange models JavaScript floating-point numbers and does not use a general-purpose theorem prover. It merges branch results into continuous ranges, which can lose precision when possible values are disjoint. Repeated calculations and transitive comparisons are among the patterns that can make its analysis less precise.
So “proof” should be understood narrowly: the tool reasons about the supported subset under its model and assumptions. Its analysis can identify useful constraints and hazards, but its abstractions may be less precise than the program’s exact behavior, and unsupported code remains outside its conclusions.
When is Freerange useful?
- Use it when numeric invariants—such as a divisor being nonzero or an index staying within array bounds—are important and expressed in code the analyzer supports.
- Use
fr --auditto inspect contracts and discover places where unsupported patterns may limit coverage. - Treat a reported hazard as a lead to validate against the program’s intended behavior, and treat the absence of findings as limited to the analyzed code and assumptions.
The available documentation establishes what Freerange aims to analyze, but does not provide a head-to-head benchmark against other tools. Its fit is best judged by whether its supported syntax and findings match the TypeScript patterns in a particular project.
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Bytes issue 506, dated July 24, 2026, presents Freerange as a response to the kinds of subtle bugs that can lurk in TypeScript—especially in code being generated or modified rapidly. The joke in “one way ticket to TypeScript hell… and back” is the familiar gap between code that looks well-typed and code whose runtime values satisfy the assumptions its calculations need. Freerange offers a focused way to reason about some of those numeric assumptions, with a deliberately bounded scope.
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Bytes issue 506 is the newsletter feature that introduced this framing.
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